One trick for simple syncing that I use is to create three tracks in Vegas. On the first two put the original video recording. On the third put the alternate audio source. Pan the audio for the original (track 2) all the way to the left channel, and pan the new audio(track 3) all the way to the right. Then using headphones play the project. Move the audio on the third track until you hear the same thing in both ears. Meanwhile zoom in and visually look to see if you can line up the two tracks better. The farther in you can zoom and line them up the better.
This works as long as the original source is in sync. Sometimes due to distant it may be a bit off and you'll have to compensate. Either way doing this will get you close to in sync.
You'd probably also want to make sure it doesn't drift out of sync. So play and listen to the beginning of project and the end. If there is a time difference you can do a simple time stretch on the original video to match the audio. To do this hold CTRL, hold the mouse button on the right edge of the original clip and slide it so it visually and auditorally matches. Then check the beginning again and readjust/repeat if necessary.
Once you have it lined up just delete track two and pan the new audio back to center. Then you can mix down.
If you want to create a multicam video use the same method. Sync one video, then add another video and audio track. Sync those to the new audio, delete the original audio track for those, and move that video track to the top above the other. Repeat.
Once you have multiple video tracks you can right click on the track button for each video (on the left w/ the volume controls) and pick Insert/Remove Envelope/Composite Level. That will give you a line in that video track (defaulted at 100% or the top) which when adjusted will mix the video w/ the track(s) below it. IE if the video track on the top has a composite level that hasn't been adjusted it will be the only video seen when outputted. If you adjust the envelope you'll start to mix in the track below it (if it's at 100%) and so on.
Rambling but I do this all the time for my DMB youtube channels (Re:
www.youtube.com/chestercopperpot5 ). Hope this helped get you started...
-Jay