I'm one of the people who had the game broken by the patch. Not only does it lag horrid now, but V.A.T.S. no longer works with a 15 sec lag and 0% accuracy always. Does anyone know if there's a way to re-install and prevent the patch from force updating via Steam?

The only way to go back is if you copied the older exes elsewhere before updating. (Like me.) I have them all copied to a backup drive for this very reason. Thankfully I have not needed to go back.

If you are using Windows 7 (and possibly Vista), try turning off the automatic updates for New Vegas (properties tab in Steam itself), then go to the FalloutNV.exe file in Windows Explorer, choose Properties for that, and check the "previous versions" tab. If you have a stored previous version you can roll back to that and keep the ESM and BSA updates that Steam previously downloaded.

If not, and you have the retail disc you can go back to the initial release version, back up the EXE from that, let Steam patch the game and then use the old EXE file.