It's actually pretty good. Vaporeon's base speed stat is pretty low, so it wouldn't be outspeeding anything anyway. So basically, yeah, if that's what you've gotten, stick with it. Are there better natures? Yes. But since Vaporeon is hard to get, I think it's an OK Nature.
I would say yes. He makes a good tank with high hp and decent special attack. You should stick with it and use an EV set like this: 252 hp/ 152spatt/104 sp def or def. whichever you prefer.