You can install Vegas (well, not only Vegas, but virtually anything) using "modified" .msi install package. You shall need a .msi edit tool like Orca (http://support.microsoft.com/kb/255905/EN-US/). When setup says "Operation system not supported" go to a temporary file storage (most common path is C:\Documents and Settings\%username%\Local Settings\Temp). Find the most recent .msi file and copy it somewhere. For clarity I shall name this "file.msi" Quit the setup. Launch Orca (or whatever) and find "LaunchCondition" table there. Modify the "Condition" field to a "VersionNT>=501" value and save transform file to a "file.mst" in the directory saved file.msi lies (Transform->Generate transform in Orca). Launch the command shell (WinKey+R->cmd) and change directory to the one your .msi and file.mst is placed. Type "msiexec /i "file.msi" TRANSFORMS=file.mst". Done.
I have absolutely no idea whether this method is legal. EULA prohibits modifying program files, but no modification to the files in question is being made. Maybe you should consult with someone. I do no know how the installed program shall work (worked like a charm for me), but you will be able to install Vegas, as you have asked.