click the clip, select properties and check out the playback rate

No.

When you overcrank a camera you capture more frames per unit of time. Slowing this back down leaves you still with many frames per second. For example, if you crank a camera to 3x you'll get 72 images every second. Slow that down to 1/3 and you still have 24fps for smooth motion. When you slow down video you end up with fewer frames that last longer. So if you slow 30fps video down to 1/3 you'll only have 10fps and the motion will be jerky.

on the contrary, overcranked video, played back at a slower rate, looks very smooth and fluid. I was asking because the overcranked footage I've seen , looks much smoother when played back at a slower frame rate than what I've been able to achieve with the velocity envelope. I don't thnk Vegas gives you the option of playing back, say 60 fps, at 24 fps.

Ahhh, you actually have overcranked video? Sorry, i didn't pick that up from your post. In that case it should work fine.

And yes, Vegas can play anything at any frame rate. To convert 60fps to 24fps set the playback rate to 0.400 and the project frame rate to 24fps. That's really all there is to it. Setting the velocity envelope to 40% will do the same thing as setting the playback rate to 0.400, except that playback rate affects both audio and video, whereas velocity envelope only affects the video.