Vegas is super fun on New Years but be warned that some of the resorts close to non-guests because of parties and other events. The strip gets absolutely packed and can get wild.
Vegas is fun at any time, very hot in the summer but still a good time to visit. I've been going to Vegas since I was a very little kid and there is never a bad time to go.
I loved NYE in Vegas as a college kid, not so much now. It will be crazy expensive and packed, like so packed you can't walk down the street. But it's an awesome party scene if that's what you're looking for.
Early to mid October is good . Weather warm enough that that all the pools should be open, but not super hot like the summer . Plus legal betting on football games. Depends on what want from your trip.