Does venom grenade stack
The -resistance does not stack.
There is however still some advantage to using it anyway for the damage. It deals almost as much damage as Frag Grenade or Supression and has a larger AoE and lower endurance cost.
It doesn't stack because it's already silly-good, especially in conjunction with Surveillance and Poisonous Ray. The two of those together do pretty substantial damage, and when combined with -60% resistance to the least-resisted damage type in the game, it can do alarming things. My Bane with just vanilla IOs and no -KB can defeat Ballistae faster than my ultra-pimped characters just by abusing these three in concert.
The debuff doesn't stack, but Venom Grenade is still a great AoE attack, and if there are multiple targets, you should still spam it for the raw damage.
Rule number six of an empathy defender is NEVER underestimate a blaster's ability to die. I don't care if he has CM, Fort, both RAs, bubbles (both FF and Sonic), and is fighting next to a Storm defender with hurricane on. If there is a way to die in that situation, the blaster will find it.
...with the same caster? My crab is perma-hasten so is it necessary to keep dropping VG every time it's up?