You record your voice as .mp3, while watching a low-quality rendering of the video. Then, you import that mp3 file on the timeline in the "voice" track. If your video already has other sounds, it is recommended that you make their volume smaller, so the voice-over sounds more clear.
You can also record directly in Vegas to a new audio event while watching the video in Vegas' preview window. Choose an audio track and click the "Arm for record" button in the track header. Then while previewing click the record button in the transport controls at the bottom of the timeline. Vegas will start recording in sync with what you see. Press stop when you're done and you'll be prompted for a file name to save the recording in. It will then be on the timeline right in sync with what you saw while recording it.