That brings up a question. What is the rule on companions? Because I had raised the issue some time ago about swapping my companion from Bashir (who I never really got any traction with) to Cecelia (the redcap). There was some discussion, but I don't know if a decision was ever reached.
Dicking Vulcanus over by taking his apprentice sounds A: highly appropriate (because Vulcanis is hardly an ideal master after his display at Belaquin, surely), B: there'd be technically nothing he could do about it, and C: a hilarious adventure, which is clearly the most important factor 
Yes and no.
Technically, you're right. Vulcanis would not have any legal standing to oppose Fleur taking his apprentice.
Buuuut, he's not without options. Consider:
- He could declare Wizard War against Fleur (for some pretense, if not for stealing his apprentice). Yes, "unjust" use of Wizard War is discouraged by a majority of level-headed magi. I'm sure that'll be a comfort to Fleur when Vulcan appears at her doorstep.
- Fast forward six or seven years to when Carmen has an apprentice she's been training for five years. Suddenly a Bonisagus friend of Vulcanis shows up and takes Carmen's apprentice away from her. There'd be technically nothing she could do about it. Yes, that's abuse of the rules. But it's possible that other magi would overlook that if Andorra was seen as engaging in the same practice. What's sauce for the goose ...
- Higher up Members of House Bonisagus might come to have a little chat with Fleur about how she's using the Bonisagus right to claim an apprentice. Who knows what pressure they could put on her?
- Some time later Andorra is before the Grand Tribunal trying to get a proposal for a new Tribunal passed. Enter Vulcanis who's been poisoning potential allies by telling them how Andorra can't be trusted, and will abuse the rules. He points to the stealing of his apprentice as evidence, and some members are swayed, finding the indiscriminate use of the Bonisagus right to be distasteful.
So yes, technically there's nothing he could do to stop the apprentice snatching. But there's plenty he (or others) could do to retaliate. And a lot depends on how other covenants will react to the apprentice snatching. This is why Fleur is being very careful to consider the political ramifications when deciding whether and when to exercise her Bonisagus right to take someone's apprentice.
Of course, in this case, she's got a better leg to stand on, given Vulcanis's recent illegal actions. But it's still a matter that requires serious consideration.
The other thing is if she does it once, and actually trains the apprentice while using them as a lab assistant, Vulcanus may try and poison others against the covenant, but really who will listen? Vulcanus is socially disadvantaged by his personality if not his reputation, and I'm sure he isn't the first master who abused his apprentice then decided to complain that the Bonisagus who rescued the apprentice was abusing their authority. Of course in any case we would first have to find the apprentice and confirm the suspected abuse (the threshold for the second part being a bit arbitrary)
Plus it isn't like Vulcanus is currently friendly towards Andorra (or Belaquin...)
Unfortunately, politics goes deep. I'm less worried about what Vulcanis does himself. You're right, he's a difficult person that people are unlikely to listen to. What I am worried about is those people who are predisposed to dislike Andorra who would use his cause as a fig leaf to oppose the covenant. They don't have to like Vulcanis to oppose what Andorra is doing; they just have to dislike Andorra.
The real question, and I don't feel like I have have a good answer for it, is what the average magi on the street would think of this. Would he care? Would he shrug it off as somebody else's problem? Would he be upset because this might set a precedent that puts his own future or present apprentice at risk? All good questions.
I see a few issues that seem paramount to the answers, though: (1) is Vulcanis actually being a bad master (or could he be cast as one in a believable way); (2) would Fleur actually train the apprentice for the full fifteen years (or the balance thereof), or would she hand the apprentice off; and (3) how will Fleur's hermetic age play in the debate?
The first issue is a relatively easy one. As you say, Vulcanis is unstable enough that a decent argument could be made that he was unsuitable as a master. It should at least pass the straight-face test.
The second is one I can't answer. Only Fleur can. If she genuinely is looking to take a new apprentice and train that apprentice for the required time, then obviously that speaks in her favor. But if she's just looking to take the apprentice for a few years and then hand her off (or 'sell' her off), well, that's another thing. It certainly looks bad, and a little bit like chicanery.
The third one is another I have trouble answering. I have to imagine that what Fleur is proposing is highly unorthodox. She's only a few years out of Gauntlet, and many older magi might say she has no reason to take on an apprentice right now. I mean, she can't even open up the apprentice's Arts on her own. Some might claim that she doesn't have the experience to train an apprentice properly. Now, I know that Fleur was designed at least partly with this situation in mind, so between her and her familiar she actually has a lot of Teaching ability. And her low Art scores are something she can fix along the way. She'd actually do quite well training an apprentice. But will most magi be able to see past her hermetic age or the fact that many of her Art scores are currently low? I just don't know. Will magi even care about this? Will they shrug it off as none of their business or will they get offended by an uppity new maga? I've no idea. I just don't have a feel for the pulse of the hermetic community the way some others do.
But these are all issues that come to mind. However, I leave it to wiser heads than my own to sort through them and come up with the answers.
Quite - hence the technically! Though really, as pointed out, if this is even an option on the table then we'd need to find the apprentice, and see what sort of condition they're in. If Vulcanis really is a decent master, then stuff gets a lot dicier, and we sink deep into politics and the potential ramifications of rules abuse. On the other hand, if Vulcanis beats his apprentice once a day, and twice on Tuesdays while keeping them chained to his forged day and night...well, I'd be inclined to say things are more easily decided at that point.
Further information required!
A few more things to consider. 1) How will House Flambeau react? 2) What about House Verditius? and 3) Maybe Vulcanus was secretly doing you a favor.
House Flambeau: Vulcanus is of House Flambeau, the guys that invented Wizard War. Not only that, but he is part of a sub group of Flambeau that relish in the old traditions from earlier editions. The Knights of Seneca. And our covenant has an ancient link to that group. What if he wants to visit the Eternal Flame? To refuse him would make us look bad.
House Verditius: The Valdarian lineage is in possession of a small set of Mysteries originating in House Verditius. The Verdi have put up with it for three reasons. First is that the set of mysteries is small and ever shrinking, confined to a single lineage cut off from the source. The second reason is that, well, you don't mess around with House Flambeau. Don't step on Superman's cape, don't pull the mask off the Lone Ranger. The small lineage with a small shrinking set of mysteries is not worth it. The third is hope. See, the Valdarians have an improved version of the Runes, and the Verdi hope to learn it some day. Thus we have Antoine.
What if it was a Favor: Vulcanus is an ass, and anything he does is easy to view in a negative light. But what if, in his mind, he was doing you a favor by disrupting Valgravin's scheme. What scheme? It doesn't matter. Vulcanus thought Bellaquin need to be taken down a peg and did so in ass hat fashion. Maybe.
Carmen is no fan of Vulcanus, in fact she hates the guy. But he is Flambeau of the Knights of Seneca, as was her father. She is a Lady of Seneca, the first female admitted to that societas. So she is not keen on the idea of snatching his apprentice. The Valdarian lineage needs to continue. As far as I know, Vulcanus is the last one. There may be another, not sure. But they are rare.
I don't care how many companions or even magi you run. I suppose it is up to the troupe. But I also really dig Bashir. He saved a whole village of sick islanders, and aided Roberto after he nearly drowned.
Maybe I need to give Bashir a bit more of a chance. 
Perhaps we should at least make the investigatory quest to assess the situation. Maybe his apprentice already knows the Verdi secrets and graduating as Bonsagus might make him consider revealing them to either the order as a whole or house Verditius. There are a lot of maybes and Fleur doesn't really need an apprentice for another year yet. When I have the new companion finished he might be the best choice for investigating the situation.
There is also a lack of clarity in what is involved in claiming an apprentice- are you required to let the previous master know, and if so in what time frame? But those are later questions...
Rilcheau - I like the name. It makes me think of Cardinal Richelieu.
Would this person possibly be a direct superior to Bernat?
He might become an immediate superior given that he is technically a magus in terms of the rules of the Order, but to begin with he is going to seek some kind of lab space and association with the covenant, given that there is a close relationship with the covenant and Mercere and there have been some political stirrings in the area, plus he wants somewhere to work on his philosophical experimentations, preferably somewhere with an aegeis in case he upsets the wrong person.
Just to put it out there, I was wondering if anyone noticed the part where Rodrigo told everyone what his Might Score is and where Vulcanus is hiding out at.
I did, but Guiverna wasn't there.
I don't suppose that is a registered covenant that a redcap might deliver mail to?
Oh I caught it, but Solomon has no great interest in going after Vulcanus.
Penetration is your friend: It improves all your spells, and once you've tasted blood, it multiplies.
As far as I can tell, only effects that affect the javelin.
Say, a javeling that can hurl Pilum of Fire, or cast Endurance of the Berserkers on you: You don't care.
Invisible Javelin, or a MuHe(Te) spell to make it tougher: It needs to penetrate.
Oh my, this dates back a week already??? 
Time flies.
Hope I'm still relevant.
I support this idea wholeheartedly
(The spider)
Nonsense. It would imply for Vulcanus to be able to get along with someone else. That's just preposterous.
We just need to follow Durenmar's noble tradition, and make him pay some fine. Say, 1 pawn of vis. His ego won't be able to stand it ^^
Tiiiired.
Hmm, that's an interesting interpretation. It doesn't really help me for what I was thinking (a ReHe effect designed to teleport the javelin to me would be an effect that affects the javelin). But still presents some other interesting ideas. I may just have to create a ReHe(Te) spell to teleport a completely mundane javelin to me (that I have an AC to), and look at making a different type of talisman altogether.
Wander over to Gunnvara's thread if you want input on her magical warder/partner in crime before it gets locked down in play. Putting this here so everyone sees it.