Stock up on balls and berries and catch ’em all!
Here’s why every grass Pokémon can be useful in gyms:
Exeggutor is one of the best attackers against Vaporeon. Also good against Snorlax and Blissey.
Venusaur is a great attacker against Vaporeon. Ivysaur and Venusaur are also great prestigers against both Vaporeon and Rhydon (they are actually the very best against Megahorn Rhydon).
Parasect with Fury Cutter / Solar Beam is one of the best prestigers against Vaporeon, Earthquake Rhydon and Earthquake Snorlax. Also good for prestiging against Blissey together with something else. EDIT: I forgot to add that Parasect with Fury Cutter / X-Scissor is a formidable prestiger against Exeggutor, thanks to its double Super-Effectiveness and to its double resistance to Grass moves.
Vine Whip / Grass Knot is the best overall grass moveset. Excellent prestiger against Vaporeon, Rhydon and Snorlax, also good against Blissey. And that at the cost of a revive and a Super Potion. It can also be a good attacker against Vaporeon/Rhydon. And it evolves into Tangrowth in Gen4.
Jumpluff is the best prestiger against Rhydon with Mud Slap / Earthquake, due to its double Resistance to Ground. Good against Vaporeon and other Rhydon, just avoid Stone Edge.
Victreebel is a decent attacker against Vaporeon but above all a great prestiger against both Vaporeon and Rhydon without Ground moves. Weepinbell is also a good prestiger in the 1000-1100 CP range.
Vileplume like Victreebel is a good prestiger against Vaporeon and non-Ground Rhydon. Bellossom is a good prestiger against Vaporeon and Ground Rhydon. Go for Razor Leaf / Leaf Blade.
Sunflora with Bullet Seed / Solar Beam is a very fast prestiger against both Vaporeon and non-Megahorn Rhydon.
Meganium is a good prestiger against both Vaporeon and non-Megahorn Rhydon.
For more details, check my CP efficiency calculator.
Disclaimer: some movesets of course just suck (Vileplume with Moonblast says hello), but there’s no way to know the moveset in advance so just catch and evolve ’em all.