So it looks Vegas 9.0 (all minor versions) in its 64-bit edition requires that you install it as the TRUE Administrator (at least under Vista x64, running the installer as regular user with Administrator's privileges is not enough).
There are several methods of running an app as the "true" admin - but probably the least known one is enabling the actual, in-build administrator's account, using the following command in the cmd window:
net user administrator /active:yes