It really works on a case by case basis. Uno and Texas Hold 'em DON'T work together. I tried it and was annoyed that they didn't at first but when you think about it, it makes sense because there would be no way that you wouldn't see each other's cards.
Two things, she could always play online in most games signed in as your guest. Obviously not in ranked matches but anything goes in unranked matches. That was always an option.
We've done that with Halo 2, but she's disappointed that she can't use a headset. Instead communication sound defaults to the TV speakers regardless of whether you have a headset plugged in or not, and she isn't given an opportunity to trash talk. Also, she's been wanting to play along with our MP Gears of War events, but you are unable to have a guest sign on in Gears. A second profile with a Live account is mandatory. I'm not sure if this is the case with Rainbow Six Vegas, as I only recently got the game and haven't explored the online multiplayer very much.
Second, still can't play from the same XBox in ranked matches, but yes you both can log in from the same machine to records stats and stuff when playing online. My buddies did this in Perfect Dark Zero.
I doubt she'll want to play in ranked matches, anyway. She's more interested in playing with friends than random strangers. Ranking doesn't mean anything to her. Playing with friends is far more important. I didn't realize you couldn't play in ranked matches with two accounts, but I guess that would allow for potential cheating, so I can see why this is not allowed.