Something related to GFWL is buried somewhere in the Vista profile that MS can't find yet. I spent hours with Gilgamesh trying different things (reinstall H2V, reset LIVE, edit the registry, reset machine id for LIVE, and so on...), in the end all it took was a new Vista profile.
I know recreating a profile can be a pain, especially since Vista has some more check and balances in place than XP did. You can simply rename your profile, Vista is too smart for that and will annoy you. Just create a new Vista account and test Halo 2 in there.
I know it worked for BlackChief and I, we had somewhat different problems. Maybe it will work for some other poor soul out there. Either way, it is a quick and simple test.
1) Create new Vista account/profile
2) Log in to new account/profile
3) Run Halo 2 Vista and see what happens