Wizard's Boost text says: "You cast this spell as another spell of a level less than this one is cast."
Does it mean that this spell is cast in the same round as the boosted spell?
If so, with what rules?
The Concentration Table on page 82 suggests that it is possible to cast two spells simultaneously, but it requires an Int +Concentration roll vs Ease Factor of 15.
Alternatively it could just affect the next spell one were to cast if that's easier for your troupe to digest.
Serf's parma, but don't the table of MuVi says that MuVi casting spells use a level 6 concentration check to cast both spell in one round?
It's also possible to cast it on a spell cast by another magus if you are in range. (But then you enter the realm of "what do I need: penetrate his parma to affect his spell? or not?" and as a MuVi-user you still have the everlasting question which is: "What is the penetration of the resulting spell? the initial spell one? the lower one of the MuVi and the original spell?")
No, but there are certain spells, the one I can think of is Maintaining the Demanding Spell which does lower the EF for concentration to 6+ to cast it while another spell is active, because it was designed to do so. There's no such wording in Wizard's Boost. The normal difficulty for casting another spell while maintaining concentration on yet another spell is 12, with +3 for each additional spell if more than one is being maintained.
Casting MuVi on another spell being cast by a different magus requires:
Let me also add that Wizard's Boost is a legacy spell. I've always been hard pressed to come up with a reason for this spell, and given the Errata that Tellus reminded us about, there's even less of a compelling reason to use the spell. It puts everything into the hands of the storyguide, and really doesn't provide guidance beyond that.
Wizard's Reach has a clearly defined purpose. Wizard's Boost should probably be eliminated or changed so that it can modify the Target of the spell, I can't recall if there's a MuVi spell that does that, if others see a purpose in Wizard's Boost, I'd be most interested.
I like the MuVi modifying spells, but they do need a decent definition of what they accomplish and also the method to use it. Another good example of a boosting spell/effect was Hugh of Flambeau in Magi of Hermes. He invested two Might stripper powers in his talisman that had low Penetration, and then put in an additional effect later on to add Pen to them. That IMO is a proper way of doing a Muto Vim spell effect, Wizard's Boost is rather ambiguous.
Maybe a better way of doing Wizard's Boost would be that when you learn the spell you pick a modification for that version of the spell you know, much like you pick a Mastery Ability whenever you gain a point of Mastery. So for Wizard's Boost there might be a standard list of effects that Magi have done in the past with various spells and you pick one or design one along those parameters. Then you can learn the spell again to get a different effect (but get a similar spell bonus). This makes it in line with some other Vim spells where you need to learn multiple copies for the different Forms or Realms.
The "Maintaining another spell" situation (under the Concentration Table) stands for "Maintaining another spell that requires concentration", right?
Yes. Spells of other durations don't need to be maintained, they last until their duration expires.
I tend to regard that as a mistake that slipped through editing - it's not part of the guidelines as presented in the corebook. Muto Vim should not be able to directly boost penetration, it breaks the game.
Edit: I do agree that Muto Vim needs fixing though, as it's far too powerful as is. (Luckily David Chart seems to think the same way, judging by his posts on the topic.)
Magi of Hermes has many problems. I'm not sure if Muto Vim being too powerful is one of them. It's an arguable effect that Muto Vim can affect penetration, since it is really in the purview of the story guide as to a lot of these effects.