Perdo Vim (General) "Create a magical shell that gives false information about the target to intelligo spells with less than half its (level + 1 magnitude) on myself in order to hide the fact that I have any active spells on me from people with Intelligo Vim and/or Magic Sensitivity skill
Yes, seems very reasonable use for Perdo Vim. I think the guideline to use is the first one on p160. It obfuscates something (including magical items) from detection as magical by Intellego spells less than or equal to twice the (level of PeVi effect, +2 mags).
Assuming you create the spell as +0 Personal, +2 Sun, +0 Ind then the Perdo Vim spell will protect against Intellego spells of twice it's level. I'd probably create the spell as +1 Touch, +2 Sun, which makes the formula: Twice (level -1 mag); which is just like Masking the Odor of Magic.
I'm not sure what equivalent level is needed to stop second sight or magic sensitivity - ballpark level 20?
My initial thought was "oh, just have them roll vs. the effective level of the spell!" - but that's more like penetration and Might. And PeVi is the inverse of that, isn't it. (ie, automatically cancelling things out, regardless of how well they roll.)
With that in mind, I'd do the x5 rule - ie, the ability is equivalent to it's own level x5. So it would take a lvl 10 spell (Personal, Sun +2; good to block out anything up to lvl 20) to block out a lvl 4 (4x5 = 20) Second Sight or Sense Magic ability.
The guideline you have quoted/referenced is Creo Vim, not Perdo. It could however work to do what you want, to a certain extent, the 'false information' being a lack of information/presence. You could also use the other shell guidelines to hide your active spells behind a solid magical aura around you. The 'shell' spell could conceal everything else about you and your active magic, though it itself would be detectable.
There is already a spell that specifically makes things appear completely mundane however using Perdo Vim, using the actual Perdo Vim general guideline for making a something appear non-magical/mundane. The guidelines and spell both are on page 160 of Ars5 Core, the spell being 'Masking The Odor Of Magic'.
Ah, good point. Yeah, I instinctively shifted over to the PeVi spell that actually does exactally this, rather than what the OP asked about. In that case - I'd go with penetration, then, for the CrVi effect. The Supernatural Ability has to roll over the effective level of the spell. Sucks to be them, but that's what you get for trying to pit a regular Ability against two Accellerated TeFo's. For the PeVi, I'd treat the Ability like a 5x spell.