If it's not asking you to put the CD key in when you go online you must have already put it in. Uninstall the game and delete the save game folder, and clean your reg of everything ubi (other ubi games will need to be re-installed - so back up your game saves), then re-install the game.
If your having the CD key invalid error (when using a legitimate CD key), there is a way to fix it.
To fix invalid CD key issue, have a look at the thread; INVALID CD KEY (FIXED) on the Rainbow Six Vegas Forum. Follow the steps I've outlined in said thread, and you should be laughing...
The later suggestion was not the problem. I reinstalled and this time it asked me for a cdkey when going online. I think it was because I didn't register an user first this time.
