should you run more than 1 sunder charm on a vengeance pally? does that even work? does it do something weird with - to res?

vengeance guves triple elemental dmg, so imo you have four options:
run all elemental sunders
run none aince you are not locked into a single elemente
run the ones for your main elements (if you use a dmg aura or smth)
run the ones you can compensate for in terms of resistance