Sadly, you can't do this within Vegas. If you have Sound Forge you can use the Effects / Pitch / Bend function to approximate a velocity envelope. You will have to do a lot of guessing and experimentation to get it to match the video, but it can be done, more or less.

If absolute sync isn't a requirement and you don't have too many speed changes, you can approximate it by splitting the audio event up into many small sections and stretching them all individually.