There is no easy way of achieving this in Vegas, As you found out masking
is the solution, but very time consuming, as you have to animate the mask
to fit the subject movements frame by frame.
Other tools are out there however to make such a process slightly easier.
Steve Rhoden (Cow Leader)
Film Maker & VFX Artist.
Owner of Filmex Creative Media.
Samples of my Work and Company can be seen here:
https://www.facebook.com/FilmexCreativeMedia