The media that palys before the menu is called Introductory Media.
Drag your file to the project overview and drop it right on the disc icon right at the top. You can also select it and hit the Set DVD Start Button just above the project overview window.
To link different media files for consecutive playback, double-click the file, then in Media Properties > End Action, set the link destination to the media you want (that’s already been added to the project overview). You can also do this using a Playlist.
Gary Kleiner
Learn Vegas and DVD Architect
http://www.VegasTrainingAndTools.com