Where can u see vaporeon in Pokemon platinum?

Unfortunately You can't find a vaporeon anywhere in the game. You can however evolve an eevee you get from Bebe using a water stone. There are alternate Pokemon you can get from evolution:

• flareon-fire stone
• jolteon-thunderstone
• glaceon-level up near the icy rock
• leafeon-level up near the mossy rock
• umbreon-boost it's happiness to the max and then level up at night
• espeon-boost it's happiness to the max and then level up during the day

u can find flareon, jolteon, and vaporeon with a trainer around route 214.

Umbreon, leafeon and espeon with trainer on route 212.

glaceon with trainer on route 217

for more help filling your Sinnoh Pokedex head to http://pokemonplatinum.net/v2/pokedex/