I can't speak for a weight belt but I own a weight vest and it is the best investment in my training I have ever made. Aside from being handy for some weighted jumping exercises I use it for all my upper body work too (basically I onyl do push ups, chins and pull ups and dips but I find this is all I need).

As far as vertical training goes I have slowed down a bit in the last 3 months but I still use it once a week for some box box jumps and low level plyometrics and have found my vertical has maintained what it was when i trained 2-3 times per week.

Cheers

Jaydub