Don't feel bad, this one throws everyone. You expect it to be an extension of the slow motion you can get by changing the media properties, but you have to use the velocity envelope instead.
You can set points on the velocity envelope and use the keyframe capability to smoothly go from normal forward speed to reverse, should you desire that effect.
One last note, when using the velicity envelope to generate reverse speed video, make sure you force Vegas to resample the video. If you don't, the video may be slightly jerky.
Sorry about that, all. The Vegas help index has been updated to include entries for reverse/backward video. The new help file will be available in the next update.
If you ever have trouble finding other topics, try the Search tab. This tab allows you to perform a full-text search on the help file. The Index is, unfortunately, a largely manual affair, and it's susceptible to these sorts of sins of omission.