Ok guys, I am looking for what you think are the best  all traditional events in the Rocky Mountain areas. I am interested in ones that allow vendors, and that have at least 300 or more shooters. Tell me which one you like best and why.

Colorado High Country shoot at the end of June in The Flat Topps. Great shoot.

As stated above, the Colorado Traditional Archer's Society High Country Shoot.  Meets your criteria, plenty of ranges set in the middle of elk country.  A great time.  June 20-22 next year.  Here is a link for info:
http://www.colotradarchers.com/high-country-shoot-info/

As stated before the C.T.A.S. High Country Shoot third weekend in June.

About how many shooters do they get there?

I think last year there were around 350-400 shooters.