I would use Toxic. I find that Toxic is good to wear some Poke's down, and Vaporeon is not exact suited for a wall, so bumping up the defenses won't do much. Plus there's other Poke's that completely out class Vaporeon in terms of a wall. However, it does make a great support Poke. Not going to provide a full moveset or anything like that, cause I'm too lazy to look up suitable movesets, I will go with Toxic as the final move slot.
Edit: Alright...I decided to provide a nice moveset.
This is the Wish support moveset from Smogon.
Vaporeon @ Leftovers
Bold nature
248 HP / 224 Def / 36 SpD
Wish
Protect
Surf
Toxic
Other options are HP Electric and Ice Beam.