Go into File->Properties and select the Ruler tab in the dialog that pops up. Under Measure & Beats you can select the tempo and time signature (i.e., beats per measure, and what note gets one beat).
Okay Garo, had to really dig this one out. It is there . . not where I thought it would be, but some where else. It is a "Project-Wide" setting and therefore amongst that lot of menus . .makes sense:-
File>Properties>Ruler [TAB]>"Measures & Beats"
Here you can set EVERYTHING relating to .. . well .. . .Measures & Beats . .. .use the "other" thread for Setting emphasis on the actual beat. You can use any audio file that Vegas would normally recognize - yeah? Make sure you DO have the correct "Rulker" for the timeline. You do this by "Right" cicking on the exsisting time line Ruler.