you'd have to record it multiple times, a slow, fast, and normal version. even then you'd only get three speeds, you can't control the speed smoothly, even with cutting the replay a lot.
like this, but with twixtor instead of scale
why would you have to record a fast version if you can speed it up in vegas?
if you need it to be inbetween slow and normal, just adjust the speed of the slowest one. not too hard