For regular users of this forum, this is common knowledge, it is also included in many guides.
Basically, unless you happen to catch one of the evolutions in the wild, which is very rare, it helps you fill your pokedex.
The best strategy is to get one vaporeon, and then let it go random, unless you are in a hurry to complete your pokedex. Vaporeon is so much better than the others, so if your pokedex can wait, 100% chance to hit one the first time and 33% after that is the best solution.