Well... YOU didn't need a room in Las Vegas. Your girlfriend did, and could easily have booked the room under her own name. Except you were trying to allow somebody else to take advantage of your elite benefits she wouldn't otherwise be entitled to...

Your motivation is common and I don't think it's a big deal, especially if the elite member is there for check-in. But for future reference you should never be booked into two hotels from the same chain at once. You won't get double credit and it looks dodgy.