While not being specifically familiar with Vegas - most video editing software has an "export" (or some wording similar to this) command in the File menu, which when you click on it will bring up a dialogue box with various settings -video file types, audio settings etc.
For Youtube settings:
"Once you're happy with your final result, you'll need to save the video in a format that YouTube can accept in order to upload. Unless you're a professional video producer, we recommend that you save your videos as either QuickTime .MOV, Windows .AVI, or .MPG files— these are the most common formats and they work well within our system. We specifically recommend the MPEG4 (Divx, Xvid) format at 640x480 resolution with MP3 audio. Resizing your video to these specifications before uploading will help your clips look better on YouTube." (from Youtube's Video Toolbox at
http://youtube.com/t/howto_makevideo)
good luck and have fun