Use the velocity envelopes and set the speed to -1
Sorry i can't be more specific, I'm only a Vegas Audio LE user and it
doesn't have video envelopes, so i'm only repeating something i've
heard elsewhere in the forums.
right click on the event and then click on "insert velocity envelope"...once you've done that you drag the puprle envelope line down to the bottom of the clip which is -100 (reverse at full playback speed)...you can use the velocity envelope to speed up video to +300% and slow down video to -100%...