Hello, ignore this answer if your problem has already been solved ...
The Sony Vegas Pro 11 needs . NET Framework 3.5 to run, then you have this version of the Framework installed on your Windows 8 ?
If you do not know search from the home screen by "control panel " . On the control panel there is the option " Uninstall a program " click this option and the next window that appears click "Enable or disable Windows features " . Open a window last, and it already shows the first option if the .NET Framework 3.5 is installed .
If unchecked then check and be connected to the internet wait for Windows to download it and install it for you .
If it is already installed and yet your Sony Vegas does not work, I recommend that you download it again from the Sony site ( it is the last update September 2012) .
Just for a test download the 32-bit version and install it in the trial version, because if it works well, is showing that the error was in how you came installing the program .