Simple way... by changing the time span of the event you wish to speed up or slow down. Drag the edge of any event left or right while hold down the Ctrl key. A double headed arrow appears. Drag left to decrease time and thus increase the playback speed, drag edge of event right to increase time and thus slow down the playback speed. A zig zag line appears across the event to indicate you changed it. You can also add a velocity envelope.
Both responses are excellent. Another reason liking about Vegas - gives me many tools to make it just right. As a new non-linear member, just have to learn those new buttons!
To avoid the 'choppy' look, resample. Right click on the timeline in version 3. Its more automatic in version 4. Also new in version 4 you can add a montion blur envelope and fine tune the effect better.