I had the same problem. On my computer the problem was caused by too many different versions of the same 3rd Party FX being installed from other versions of Vegas. After I uninstalled many old FX, the program opened OK.
If Eric's link does not help you, which exact version of Vegas do you own now ? Example: Are you using Movie Studio 15 or Vegas Movie Studio 15 Platinum or Vegas Movie Studio 15 Suite ? And which exact older versions of Vegas do you still have installed ?