What you're describing is accurate for pve, but casts are always interrupted by damage in pvp unless its 100% absorbed by a shielding effect or equanimity/surecast is up (was stated in the patch notes when they released wolves den). Vitality only plays a role insofar as stoneskin scaling with it. Stacking it was more to survive melee lb and doesnt really seem to be worthwhile as a healer in feast since lb, if it even gets time enough to build is going to be used on whoever is carrying medals.

You're right about cast speeds, single target heals have a base cast time of 2 seconds as opposed to 2.5 for most other things

I'm not sure what you guys mean by standard action but the OP is essentially correct unless Im totally missing something. 1st stun is 100% duration, 2nd is 1/2 duration, 3rd is 1/4 duration, 4+ is fully resisted. Resets 60 seconds after last successful (ie non resisted) stun. Being able to stun indefinitely would be kinda crazy