I have a player who appreantly delights in causing me headaches.
At our last session, he asked me about the possiblity of taking a 'Mental Construct' lab as a virtue. I don't see much a problem, but wise from experience, answered guardedly.
We've exchanged a few mails, and all seemed reasonable, but now he dug up this goodie:
can I get some opinions on this?
Sounds like you're entering the realm of dream magic, as that is the only place I've heard of where you can take something mental and make it real.
Also - what is he extracting the vis from? The mental construct lab can't affect any external thing, so you can't really draw from any outside aura... Offcourse, he might be able to drain his own gift... (this sounds like an intresting part of original research - with the danger of destroying/damaging the magus' gift).
The Covenants book says 'He probably also cannot create vis in it.'
Probably is a nasty word, so I'd say go with your gut feeling. If you think it sounds like a pain, then say no.
But since you asked for opinions: I'd say any vis created in it would be like the dreamstuff vis as per Mysteries Revised Edition. Handy for use in mental construct type environments, but totally useless once you're back in the real world. Thus he has vis for any lab activities -in- his mental construct (including studying it), but he can't use it outside his mental lab for making items, etc.
That said, if he is really keen on the idea then introduce some ancient non-Hermetic secret that allows such things to be done and make him earn it. Drag him out of his (mental) lab, force him to endure trials, force him to confront dream-creatures that are also able to get into his mental lab and cause trouble there. Magical spirits that can get others into his mind-construct provide a vehicle to get others involved - and of course he may have to penetrate the mental construct of some ancient mystical being in a faraway land to even learn the secrets to begin with.
If he is that keen... he may, instead, just decide that he can't be bothered and go with the old fashioned approach.
I'd say that if you let him create vis, then in addition to studying it in his mental laboratory, he could also develop a rego Mentem (vim) spell to transfer it to an item stored in his memory palace (as per normal vis transference rules) and from there use it to boost spells or cast rituals. I advise this this because I always thought that transferring things between one's memory palace and a mental laboratory was darn cool, not because the rules say you have to. (memory palaces are a byproduct of studying the mundane art of memory detailed in Mysteries revised)
Allow it at half normal effectiveness perhaps? And without the bonus(or interference) of any auras?
Perhaps without moving it to a memory palace it disappears after a relatively short time, because of being only "just something made up"...
Well there´s certainly a lot of potential ideas for it.
Yeah, vis sure. You can't extract it from the lab, though, so it is only useful in creating magic items that improve your lab. I see no problems here 
Xavi
I think that you should defiantly consider how much fun it would be for the player. The idea of mental vis is cool but it's also clearly less useful than vis in the real world. Don't slap the player's ideas down for no real benefit. If he chooses to go with mental vis he's already gimping himself.
I would tend to allow this, as the idea seems cool, but with some stipulations:
[/]This is just beautiful!
Besides
I could see him go for this, maybe.
I just saw this quote from the player
I'm thinking about that spell, the one he brushes off as about level 5 or so. I see it as changing the thought into a physical thing not just moving the already existing thing into the physical world. The most applicable guideline is muto mentem 25 "make a mind or spirit solid (requisite the form of the shape)" This is the guideline used for "inmost companion" (p 149). Now a pawn or a few pawns of vis are not a mind they're thoughts within a mind. This seems much simpler so I'd lower it by a magnitude to start with (more if the final spell seems too tough). It will need requisites in both vim and the form he wants to incorporate it into but these are necessary so don't adjust level.
Here's what I come up with:
level 25 or 30 doesn't seem to harsh to me I think that range personal is appropriate despite the fact that the vis appears outside of the person but you could argue for touch as well. I can see the possibility that coupled with an appropriate intellego mentem spell to perceive the vis in another mind, a magus could cast a version of this at a range of touch or greater and steal someone else's mental vis.
Ouch! Thats just way too harsh i think!
What would the Aura rating of a Mental Construct Lab be anyway? I would think it's zero. So using the lab will get you less Vis, Warping, and, thanks to the Mystery of Dream Magic, possibly less security. I'm not seeing much of an upside. If they want to punish themselves because of role-play, cool.
It would be difficult to convince me that the mental lab isn't in the same aura as the magus, but I will agree that there isn't much of an upside to a mental lab.(-5 upkeep and +3 bonus for mentem are there and it wouldn't be hard to get stuff like palatial, flawless tools and flawless equipment.)
In fact, if there's no magical aura in the mental lab, then by RAW you can't extract vis at all:
The magi in my saga are just now finding out why Missing Aura is a Major Covenant Hook....
To comment the above quote first:
I think the guideline is applicable, although vis from a mental lab is IMHO clearly a Part, so I'd just use said gudeline and demand it was Part (or group if more pawns were used).
Regarding the original question:
I think this sounds very, very cool. And I'd allow it
I think the Aura in the Mental Lab would be like what is is where the magus actually is
If there is an Aura he should eb able to extract vis
When the Covenants book says it 'propably can't be done' I'd perhaps just make it a minor Mystery to initiate, something tied to the Memory Palace
Vis extracted thus can be used for study or for making items to be kept inside the mental lab - e.g. for improvements and traps. If transported to the Memory palace it should be usable for spellcasting and rituals. Making it physical would make it usable for outside use.
I wouldn't penalize with any additional Warping or such
Aura rating of mental lab = Warping Score of magus 
interesting idea, care to make an argument for it?
The Warping Score reflects the degree to which a magus has become "associated" with the Magic Realm and how close he is to becoming a creature of Magic (Final Twilight). (Unless a Merinita with Warping caused by Faerie - in this case the mental lab could have a Faerie aura!)
Mental attributes already regulate a mental lab to a certain extent, e.g. the Size is capped by Int. So the more "abnormal" (i.e. Warped) the mind/spirit is, the more abnormal the environment of the lab. Makes perfect sense to me. 
I would also have Twilight Scars manifest in the lab in some way. Say, for example, a magus has a (bad) Twilight scar where all plants wither at her touch. "Looking" through a window in the mental lab, she would see a dead forest...
The bit about the twilight scars is certainly trés cool, I think I'll steal that.
the bit about warping being aura... might be able to sell that.
If the warping is your aura, does that mean that each extraction removes a warping point?
Extracting drains an aura...
My suggestion is to use the characters own gift as the aura. You could let the aura be equal to the form resistanse of vim (that is vim score / 5 round down). And then let the character roll a stress dice - checking for botch... Any botch results in a damaged gift. (typically a flaw, as well as warping).