Worth noting that the VP8 installer would not work on my copy of Windows x64.
It spent ages downloading two versions of the .NET v3 architecture, one x86 the other x64, and attempted to install the x86 version first. When that failed - OS not compatible error - the installer exited.
If it hasn't already been mentioned, the work-around is to go to Microsoft Update and download and install .NET v3 and any patches from there and then install Vegas Pro. That seems to do the trick.