Well, there's no need to go past Sonic Foundry's own video paint program, Viscosity. It's cheap aswell at less than $100.
Although if you want something sofisticated, I'd suggest Commotion Pro 4.0, as it has the best motion tracking and rotoscoping tools. Expensive at $999, but worth it very much. It's great for compositing aswell, but the interface isn't as quick as Vegas Video's interface.