Gen 8 NU
Vaporeon @ Leftovers
Ability: Water Absorb
EVs: 252 HP / 216 Def / 40 SpD
IVs: 0 Atk
- Heal Bell / Toxic / Ice Beam / Roar
This set is similar to the one on Smogon, but with a another option that's not mentioned and the EV spread is different. The difference is that this has more than 4 special defense EVs. 40 Special Defense EVs with a Calm Nature makes sure that you don't get 2HKOed by a +4 Dark Pulse from Blastoise, as shown in this calc:
+4 252+ SpA Blastoise Dark Pulse vs. 252 HP / 40+ SpD Vaporeon: 196-231 (42.2 - 49.7%) -- guaranteed 3HKO after Leftovers recovery
Scald is pretty good STAB that most Pokémon in the tier don't really enjoy switching into. It's 30% chance to burn can help with its checks that use physical moves like non Rest Guzzlord, Decidueye, and Dhelmise. Wish provides good healing for you and your teammates if they need recovery since Vaporeon has a high HP, meaning that you can give your teammates big recovery from Wish. Protect is useful for scouting moves from opposing Pokémon and can guarantee that you get recovery from Wish, and also allows you to get a free turn of Leftovers recovery. Heal Bell is really useful that cures you and your teammates of status conditions, meaning that they don't have to be crippled with status like poisoning, burns, and paralysis. Toxic can be used for Pokemon who are immune to water moves like Heliolisk and Mantine. Ice Beam is an option to prevent grass and dragon types to come in on Vaporeon for free. Roar is another option that can stop sweepers from setting up on you, and is very useful against Sub Shell Smash Blastoise and can even threaten it with Toxic Spikes support.
You can refer to the link that was mentioned earlier, but there is one teammate I would like to mention:
Xatu is a good fighting resist thanks to its typing that can help with your matchup against Toxicroak, a Pokemon who can literally switch into Vaporeon for free. Xatu's Magic Bounce ability is useful for catching incoming hazards and status conditions, which is useful so Vaporeon doesn't have to worry taking chip damage from Stealth Rock. Xatu also has Teleport, which can be used to bring in Vaporeon saftley.