If you are looking to make it slow down over time, like a VSO, Vegas can't do that. If you want to create that effect, you can use the Windows Media Recorder to record while you slow down the audio using the scrub switch in Vegas.
Are you using V5? earlier version?
If you have Sound Forge, you can do this by right clicking on an event in Vegas, and select "Open Copy in Sound Forge". Within Sound Forge, you would use EFFECTS>PITCH>BEND and this will put a velocity envelope over the audio you have selected. Save the changes in Sound Forge and then audition it in Vegas.