A magi has the Mercurian Magic virtue, but does not have much interest in metamagic, therefore has a low MuVi total. How does that work with Wizard's Communion and Wizard's Sigil ?
Let's say that the magi knows Conjuring the Mystic Tower (CrTe35), therefore, he knows Wizard's Sigil at level 35. But his MuVi casting total is 15, including every bonus he can muster, therefore unlikely to be able to cast, even with fatigue, Wizard's Sigil.
How do you adjudicate the situation ? Can he cast Wizard's Sigil at a lower level (assuming other magis provide assistance) to contribute ? In this case, which level ? The level of the lowest ritual he knows ? or the lowest level he can cast ?
This exact situation happened when I made the Faith & Flame Toolkit. Damascos has Conjuring the Mystic Tower and Mercurian Magic, but his free spell is too high for him to cast.
In the Storyguide Booklet, I propose a house rule that he can cast Wizard’s Vigil at any level up to 35.
I agree with Vespasian. Honestly, Vim is an art most wizard's shouldn't neglect, though I suppose a Deficient Muto / Mercurian Magi combo isn't out of the question - it's not a character I would necessarily build.
To me, getting free Wizard's Vigils is a bonus, but not a very strong one. Mercurian Magic is far from being OP, so I wouldn't make it too hard on my players to benefit from it. I think at the least, a character should get a Wizard's Vigil at the level of every Ritual spell they know (because why would you get that in play but not before?). I like the "don't worry about it" house rule, it makes things easier for the player.
Yeah, at the very least you'd expect a character with Mercurian Magic to have Wizard's Vigil at the level of each ritual spell they have, because they'd have earned them as they learned those spells (on the assumption that they gained them after the virtue, and in ascending order, both of which seem reasonable assumptions). Like some things play out differently after character gen, but it'd be very weird for something that creates such an obvious and explicit disconnect if it doesn't happen.
I don't think anyone is challenging the idea that you learn a new variant every time you learn a ritual, the question is about whether you should be able to cast the highest variant regardless of how bad you are at Muto Vim and how bad the local aura is, and without risk. It isn't especially hard with Mercurian Magic to learn more than one Vigil - you rarely pick the virtue on a character that will just have one ritual to cast.
Based on several sagas I played, the only occasion magis would ever consider investing time in learning Wizard's Sigil would be to cast a high level Aegis with the required penetration. And it was done on ad hoc basis, when the troupe thought they were under powerful magical threat.
So I am currently on the "generous side", even considering Mercurian magis to have the benefit of the sigil without needing to cast it - I believe it was the case in an older edition. Because I have seen so rarely magi learning Wizard's Sigil/Wizard's Communions in significant number to allow really powerful effects.
From a purely mechanical point of view, the magi need to be both good at MuVi and the relevant TeFo to learn a high level ritual benefiting from the Sigil. This double specialisation is not frequent, at least not frequent enough in my game to be considering that free uses of Sigil would be broken.