Vaporeon > Wailord in every way unless you're making a heavy team or fat team or something >.>
K so stats;
Vaporeon - 130/65/60/110/95/65 (HP, Atk, Def, SpA, SpD, Spe)
Wailord - 170/90/45/90/45/60 (Same order)
Wailod has higher HP and Atk, while Vaporeon outclasses it in everything else.
Yup, Wailord has rubbish defenses. While it's HP is massive you have to bear in mind that is mitigated by bad defense stats- and don't forget, Vaporeon's HP stat is also pretty damn big at base 130. Vaporeon has much better defense capabilities overall, as well as access to Wish and Protect - something which Wailord really doesn't have. High HP on Wailord doesn't make up for that.
Offensively based, Vaporeon wins with an excellent special attack stat. Wailord's higher attack stat is meaningless, since the likelyhood would be you going down the special route, due to Wailord's best move, Water Spout. The fact is, without high powered moves Wailord fails to do much damage. On the other hand, Vaporeon's excellent special attack stat lets it function as an excellent offensive tank (Hydration Tank anyone? :D?) and it's ability, Hydration when used under rain lets it heal off any residual damage with Rest and wake up immedietly after.