Vis Infesta really isn't that dangerous. Basically, if you do botch with it, it's just worse than regular botches. The real trouble with it is the personality trait issues from working with it. I can see magi using it without serious hesitations; if you have a few rituals you know and have mastered, you just use up the Vis Infesta as soon as you get it and save the 'good vis' for the long term projects. It's also likely the most common, showing up in the corpses of corrupted beasts (not demons) and very low-end demons (which they probably nuked with Demon's Eternal Oblivion, destroying the vis anyways). Consider this playing with fire; use the proper precautions and you probably won't burn down anything important.
Vis Sordida is fairly nasty (3 botch dice per pawn!) and probably should be avoided, though getting +5 Casting Total per pawn could theoretically make a huge difference in a fight. In a life or death situation, a 10% chance for a potential botch is a small risk if you need to bust past some high Magic Resistance. This is the vis that comes from vis sources in Infernal Auras and mid range demons. A Vis Sordida source is a troubling dilemma for magi, as it represents hundreds of pawns of vis over time. It's not sensible to use it regularly, but it is tempting to keep it there for emergencies. Vis from demons magi defeat is also likely to have Vis Sordida, and yes, I can see taking it to use to fight your way out of wherever you got it. It's kind of like playing with dynamite, but people do play with dynamite.
Vis Prava is ludicrously dangerous and the main reason magi would avoid any Infernal Vis at all ("How bad is this vis?"). Potential botches every time you do anything with it? No thank you. Only Chthonic Magi would even think to use this stuff. It only comes from high end demons and powerful Infernal Auras, however, so it's the least common, but the very threat of it is enough to put magi off using anything tainted. This is juggling nitroglycerin territory.
The real danger here is not knowing which type of vis you've got your hands on. If you haven't sourced the vis yourself, you have no idea how bad it is, and you should default to throwing it away.
I don't know off the top of my head if the various kinds of infernal vis are particularly well known. Perhaps I'm oversimplifying, but it seems that vis from a demon has a broad reputation as dangerous and corrupting. I'd think Faerie vis, similarly, has a reputation as weird and unpredictable and likely to attract Faerie attention.
Of course, maybe there are very differing opinions.
According to RoP: The Infernal, Hermetic magic cannot detect the taint of infernal vis, but 'cautious magi with Infernal Lore' are aware of the taint and its divisions - though again, being able tell which is which isn't going to be easy. As I mention in the other thread, it's entirely likely that vis sourced from the Infernal will always register as 'unholy' even if it's no longer dangerous for hermetic use, so figuring out if vis is infesta, sordida or prava is a matter of judging the source of the vis.
However, there are probably plenty who will take the vis despite knowing that it is dangerous. They will, of course, be fully confident that they will only use it in a "real emergency", or will have instead convinced themselves that they have a clever way of mitigating against or otherwise defeating the danger. This is temptation. This is how the infernal works.
You can always take the infernal vis and then lauder it by trading with the Redcaps.
Depends on the mage honestly. Some Chthonic Magi are more likely to use it and there are always those that are arrogant enough to basically tell hell itself it's too weak for them. On the other hand I think the vast majority of them would avoid it like the plague.
Why would the majority of Magi think that? It seems to me, the skill to cover that would be Infernal Lore, and that's not easy to get. Heck, just asking around is likely to get you in trouble. I think it's a case of the Order being (Correctly in my eyes) so worried about Hell that they don't share ANY information about Demons, so most Magi are going to be SOP on Vis recovery. We know better, because we have read the books. We, perversely, have move "Infernal Lore" then our characters do......
It's a well-known bit of magic lore / magic theory that vis can be tainted by the realms. Even if that wasn't known, it seems an obvious bad idea to use refuse extracted from the "corpses" of demons as spell components.
Tracking it sounds like a challenge. We often pool into tokens, so I suppose a die roll for the surprise is in order.
Now I'm wondering if there's some kind of quality check at the Redcap central clearing house. I'm thinking not, and there's certainly a lot of vis that just gets traded hand to hand even if there is. There should be a check (vis taint is a known issue) but I suspect it isn't followed very closely.
It's a dick move, considering the magus probably paid double for the vis he's getting. It's something you inflict on an NPC and let the PCs figure out what happened before the Redcap can trade away more of the horribly dangerous stuff.
I get what you are saying --- don't pointlessly screw the PCs.
However, I think, it's better from a story perspective if things like this do happen to the PCs. It's more interesting/motivating if stories are driven by things that happen to the PCs rather expecting the PCs to solve NPC problems.
It's not pointlessly screwing the PCs if the idea is to trigger a story about the PCs solving the problem.
Whether or not they're screwed really depends on how many botch dice they roll. Doesn't matter how nice you're being, 8+ botches is going to suck.
On the other hand, I guess having them get some amount of tainted vis is a good story, and they could probably take the person to Tribunal for it (if they're the justice-y types). Hell it could make for a good story about some magus being tempted/corrupted/DEVIL THINGS.
I don't mind giving them a botch, but I would definitely avoid a permanently damaging one. Just botchy enough to amuse the players and tell the magi "hey, that vis you got from the Redcap was fouled."
There's a difference between screwing the players in such a way as to fuel a story, and screwing the players in such a way that they're hosed with no recourse.
There isn't a check because there can't be; Limit of the Infernal. (There may be a handful of Quaesitores who have Sense Holiness and Unholiness, but they're not usually stationed at Mercer Houses on the off chance somebody's passing bad vis.
I was initially thinking if the PC magi are greedy enough to keep the infernal vis, scared to use it, and devious enough to trade it through the Redcap network, if the Redcaps discover it they may not even make a case out of it. They would simply mare sure to send to the PC magi the vis prava they've had in the 'untradable' pile. Not as revenge - because they would be petty - but just to make a point. "Oh, we're sorry - we though you liked infernal vis, since you traded some to us in the first place!"