The Ars Magica Muto Vim guidelines in the official errata state that a MuVi spell only needs to be present for the length of the spell casting time, not matched to the duration of the changed spell.
I’d not realised that rule was in place, and especially the implication that it has for ritual spells which can take hours rather than a moment to cast.
Clearly then the intent is that the Muto Vim spell alters the way the magical effect forms before execution. This makes a kind of sense to me from a programming perspective; this alters the script just before it is run.
Note to self – this is about as meta as Ars Maguca gets, I’m enjoying the banter in the forum link. Openly acknowledge how bizarre this post would be to a non ArM player.
Happy spell casting folks.