I played Fallout 3 for hours, and noticed a lot of mouse lag. Recently did a quick search, and found a VERY quick fix. In New Vegas, and Fallout 3, Vertical Sync being disabled is not saved, you have to go in and manually do it.
Config file name/location for FO3: C:\Users\username\Documents\My Games\Fallout3\FALLOUT.INI
Config file name/location for FO3/NV : C:\Users\username\Documents\My Games\FalloutNV\FALLOUT.INI
Change iPresentInterval=1 to iPresentInterval=0
Hopefully this helps someone out.