Ways to change speed in Vegas:
be sure to disable the resample

for all of these...
1. right click the clip and choose "Properties" and change the playback rate.
2. right click on the clip and choose "Insert Envelope" and then choose "Velocity" and a pretty green line will appear on your clip... move the line up and it goes faster, move the line down and it goes slower..
3. hold the "CTRL key" and resize your clip by dragging the side until its the length you want it and it will automatically adjust the speed.