I know this is from a few weeks back now, but did you get anywhere with this problem?
Uninstalling .NET etc AND all those Sony apps as specified by Sony Support and posted by Mark seems fairly drastic. Surely the installer is just looking up some information somewhere to see whether the app was installed before or not.
I had a peek at the installer using ProcMon from sysinternals, (an excellent tool for this kind of thing)
Now, I know you said you crawled through the registry and Deleted everything with sony or vegas (by the way, that wasn’t a great idea if you have other sony apps on your system), but did you only delete KEYS with “sony” or “vegas”, or did you look at all the values as well, either way,
You might want to take a look under the
HKLM\Software\Microsoft\Windows\CurrentVersion\Installer\UserData tree.
Search for Vegas there and remove and delete the parent of the InstallProperties Key in which you find it.
On this system it was in
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\F80687B3A90DFD115AE400313D6D9992
Folks, This is a really simple registry edit, so don’t be scared off, but at the same time, if you are not comfortable poking around in the registry, please don’t screw up your system and then blame this post.
Let us know here how you get on.