When you are in Vegas next week tell the consultants at the venue your guestlist situation, that you'll have a maximum of 100 but a more realistic number of 80 give-or-take. If the venue has a good group consultant they'll have plenty of ideas with how to deal with fluctuating numbers. As the PP mentioned, it is always best to reserve your room as far in advance as possible, Vegas is such a popular place for weddings and conventions alike!
Good luck!!