I know most you you all hate HobbyTowns, and I'm not sure about other HobbyTowns other than the one I go to, but they stock most Serpent Kits, including the Vector. They also have an entire section dedicated to parts and upgrades specifically for those models.
Me, I've got the Serpent Impulse and I love it. The dang thing handles like it's on rails, and it's quick as hell with the stock NovaMega side exhaust .12. I think they all come with rear exhaust now, if you get a kit that comes with the engine.
The Serpent is also the most difficult kit I've put together. It took me hours longer to do the Serpent than any other I've messed with, but the quality, fit and finish are incredible. I'd reccomed the brand to anyone. I've never messed with the Yokomo or Mugen, but from everything I've read, they're also great choices.
Good luck with whatever you choose.