Scrambling a spell seems like PeVi to me. While this spell would be less effective than the normal PeVi stuff at just stopping a spell, it includes the possibility of the other caster botching.
Personal feelings aside, there is nothing in the Rules preventing a MuVi spell from being used to disrupt a spell defensively.
However, according to the MuVi Guidelines (par iv) , any such spell would have to be (successfully) Fast-Cast to have its desired effect. And it must beat Penetration- so those together may be a practical limiter.
Agreed - by definition, the spell has been significantly changed from "as intended".
Hm....I've been thinking about this one for a the past hour or so, and I've come to the conclusion that you can't actually, directly, consistently, cancel out magic with Muto Vim.
The fallacy I was falling into was thinking of Muto as an actual Mutation - which, in modern science, is BAD for the subject 99% of the time. (The other 1% allows you to shoot laser beams out of your eyes.) But Muto magic is simply a change - if not for the better, then at least not for the worse. And changing a "living" spell to a "dead" spell isn't Muto - that's Perdo. OK - but you can "inconvenience" the spell to the point where it doesn't do what the original caster intended. For example:
Changing a target from Individual to Circle - actually a useful technique, if you're a ward specialist. But if you fast-cast it, then it becomes an effective countermagic. But it only works on T:Ind spells. (and only those within a given form.)
A reverse wizard's Reach might work (EDIT - as Toa seemed to have already mentioned) - one that removes a range magnitude. But it might not consistently work, as you might still be in range.
Changing the Form of the spell to something that doesn't affect you might also work - but you're left with odd consequences. (What is the Creo Vim form of Ball of Abyssal Flame, anyway?)
EDIT - Hound, my reasoning was that causing a spell to fail is analogous to causing a body to fail - which is Perdo Corpus, not Muto Corpus. However, changing a body to the point where it's "effectively" failing can fall into Muto Corpus - such as the current thread on making someone so fat they can't move. However, the 'failure' aspect is a side effect. Can you think of a Muto effect for a spell that would functionally cause it to fail? As Toa mentioned, the best one I can think of is a Range reduction.
Whenever I delve into a metamagic concept magus I always wish i had a spell to send the opponent's magic back in his heir own face. However with Muto Vim this seems almost impossible in practice since it is likely to be a significant change.
Changing the target (lower case 't) of the spell could do it. As could reducing the Range (upper case "R") to Personal.
I could see it as a ReVi spell, using a guideline on par with "Sustain or suppress a spell cast by another with level less than or equal to half the (level + 5 magnitudes) of the Vim spell." (Probably with a "level + 3 magnitudes" instead of +5.)
It would need to be timed much like a MuVi spell (so probably fast-cast) and would need to be R:Voice at least.
Yes
But suppress sounds like temporarily freezing the spell, or rpeventing it from affecting anything. Not really what I had in mind - I want to stab him with his own sword, so to speak.
Also, for D:Mom spells how does momentarily suppressing it work? Most likely preventing the spell doing anything, and having sort of the same effect as an Unravelling spell: The target spells fails to do anything worthwhile.
These things aren't that hard.
But deflecting the BoAF back into the face of the caster - now that's awesome (and hard).
I'm not suggesting using the same guideline, just one similar to it in terms of parameters -- that why I wrote "on par with" (emphasized above).
It would be a new guideline, probably worded something like "Redirect a spell cast by another with level less than or equal to half the (level + 3 magnitudes) of the Vim spell, so that it affects a different (but still valid) target, which may include the original caster."
Considering the need for R:Voice, it means that (if my math is correct) you would need a level 35 (Base 25, +2 Voice) spell to swing back a Pilum of Fire.
This is already a MuVi guideline in the core book. And, despite a comment above, I don't think it's much harder than canceling the spell it's retargeting, and it should probably be a little bit harder anyway since it is effectively more powerful.
If the spell is rebounded, what happens when it hits the resistance of the original caster? Which penetration total is used? The original penetration by the original caster? The penetration of the MuVi spell? A difference of the two, showing that overcoming the caster's penetration took a lot of "english" off the original spell, meaning not much is left to get through the original caster's MR. And is that difference measured against the original Form of the spell or Vim?
To use the BoAF example, I like the idea of someone's MuVi spell redirecting it at the caster, and the remaining penetration of the spell being used against the original caster's Vim resistance. It's like he's finding a chink in the caster's armor. It's what, a 40th level MuVi spell to redirect a BoAF back at the caster? There's not going to be a lot of leftover penetration in most cases, IMO.
Isn't that just this thread again?
Or maybe this one (see especially the poster just below the linked post :p).
You know, it just might be.
I did say "whenever" , meaning it happens again and again 
Good point 
I'm sorry... just no. A thousand times no.
The "Anything "worse" must be Perdo" paradigm is so flawed I don't even know where to begin. It's not that simple, nor that restrictive, and I truly wish people would stop applying such an over-simplified and blindly narrow interpretation of one aspect of that Technique's definition to anything and everything that could possibly be interpreted as changing something "for the worse".
Change a boxer's hands into soft fluffy pillows - very bad, much worse, very fail - Muto.
Change a sword into a feather - very bad, much worse, def fail - Muto.
Change a person's body to liquid or stone - fail - Muto.
Change a spell so it isn't effectively what the caster intended - Muto.
This isn't "killing" the spell any more than turning a body to stone or water is killing the person, or turning a sword into a feather is "destroying" the sword. The person is still quite alive, the spell is still quite active, just both so different as to be unrecognizable. Almost certainly "worse", and inarguably Muto.
No, but changing a living person into a dead person IS perdo.
Thus, without specifics, "living spell to dead spell" is perdo vim. As I said, "changing a spell so it's so inconvienent to the caster is muto" - but it's a side effect of the change. Just cancelling it out, without any specifics other than "it's now broken" is Perdo.
Hands to fluffy pillows? Of course it's Muto. The defense to being punched is a side effect. But that's a specific change, and the boxer could still kick you (or smother you, for that matter). So it's not a complete defense. Similarly, you could muto his legs to jelly - thereby making him immobile. OK, fine - but the equivalent effect in Muto Vim is an Inverse Wizard's Reach (-1 to range). If you're within shouting distance, you will still get hit. So it, too, is not a complete defense.
Change the boxer into a statue? Sure, perfect Muto defense against being attacked by that boxer. But that's a side effect. Therefore - what's the change that fits within the muto Vim guidelines that allows for that perfect spell defense? IMO, Wizard's Fork + a decent Parma comes closest. But just saying "turn spell that works into spell that doesn't" isn't Muto. You have to be specific, and gain the benefit of the defense as a side effect.
This is a fair point, which is not really adressed by the spell at this point.
What might be missing from it (to satisfy Kevin's argument) might be how the target spell is destabilized, and the effects on it if the caster succeeds in controlling the spell.
What if, for example, the MuVi spell were to destabilize the target spell by injecting more power into it, the result of retaining control would be that the spell ends up as more powerful or distorted in some way. The actual effect might be up to the storyguide (or random) and would be consistent with things that MuVi can do (increased R/D/T, more damage, added penetration, change of Form).
One I can think of would be (to use the "change boxer into statue" analogy) would be a sort of "freeze spell" effect. That is, the spell is literally frozen in the air, but thaws out later. Effectively, it's forcing the spell to accept a ReVi "Delay effect" watching ward. And, if the delay (say, Sun) is sufficiently long, the target (ie, the mage) could vacate the battlefield, and have all the spells fizzle out due to lack of an in-range target.
I would say this is probably be considered a "significant" change. (ie, it's not changing the TeFo, but it is making the spell significantly more complex, as it now has its own built-in watching ward.)
Alternately, the spell itself is simply "redirect into a pre-existing watching ward" - in which case it would probably be superficial, but you need to have a watching ward waiting to "catch" the spell.
EDIT - or, alternately, it might simply be what happens when you fast-cast a Watching Ward with an uncooperative magi.
EDIT II - yeah, now that I think about it more, this is more likely a straight-up ReVi effect - It would likely use the same guideline as a normal Watching Ward, but would have to beat the Magic Resistance of the uncooperative spellcaster, would have to be at voice/sight range, and would have to be fast-cast.
Ah, well - it's still a good idea, IMO - if you've got a ReVi specialist.
The specific effect I have in mind now for the spell is to alter it so it is cast with zero Penetration. Same target, same all of it, but zero pen. That'll make any spell cast with this effect bounce of parma. I was hunting for a way to make a spell not effect my magus and that'll work.
edit: Just needs a funky name now. Blunt the Conspicuous Spell, hmm. nah.
After reading the comments here I think that is specific enough to be applied, and also not a monumental change in the overall spell.
Disperse the Excess Energies? Penetration is achieved through leftover energy not used for powering the spell, after all.
Or maybe if you make it a General effect that reduces a certain amount of Penetration you could call it Stifle the Advancing Spell's Passage.
Hm. Well, halving penetration is basically Wizard's Fork, and that's keeping the total amount of skill rolled on the table (sort of - split between two effects). Although the benefit seems to be that it doesn't need to be based on a Form.
Personally, if I were to be ADDING penetration to an effect, I'd use the general guideline of "significant change" equals +5 penetration, while "total change" equals +10. Although thematically, magnitudes do seem to represent something different than just a flat-out skill roll.
Regardless, I would argue that REMOVING penetration would be based on a similar level of difficulty. Although I'd argue that, in keeping with the Wizard's Fork effect, you wouldn't' have to have a Form-based technique.
Fair call Kevin, I agree. if picking between trivial, significant, or total change to a spell; it feels like the middle one.