What's actually required to dispell a Perdo Vim ward? An example: (for sake of argument I'm going to assume the stress die always rolls a zero in all cases because I care about the mechanics of the spells, not the die rolls)
Ward of the wary wizard:
PeVi 20 touch/ring/circle dispells any vim spell of level 20 or less inside the circle
Can I dispell this with:
Whisper of mundane silence
PeVi 10 voice/momentary/individual dispell any targeted vim spell of level 20 or less
my first instinct is yes, the ward is level 20 or less. However, the ward is also presumably coterminous with itself, so when we attempt to dispell it our level 10 spell is entering its area, and should therefore be dispelled and fail to dispell the ward! so maybe we need a level 21+ spell to dispell the ward?
Answering that, how, if at all, does your answer change if we merely want to dispel a section in the center of the ward using a level 15 version of the second spell with part target?
Generally whenever two spells that both "could/should" work are in conflict I would compare the penetration scores of both spells with the higher penetration winning out and that spell taking effect.
That's a reasonable ruling, has support from the muvi guidlines, it does tend to favor the lower level spell though which is maybe a little strange imo
I think that is the underlying logic/rationale of the 5th edition take on penetration. It is easier to execute lower level spells well, and this carries through in multiple places in the rules. Higher level spells are "bigger magic" and can thus have much more significant effects and scope, but they are harder to do. Especially harder to "do well" as it were.
With circular wards and with other container Targets like Structure, the circle itself would be considered inside the circle. So for consistency with those I would rule the incoming PeVi spell must be strong enough not to be dispelled by the circular PeVi. That also makes things really clear. Of course, you could dispel this with a lower-level PeVi by starting with a version of Suppressing the Wizard's Handiwork to make sure the incoming PeVi doesn't get stopped.