Hey I have been trying to figure the same thing out, only key I still need for the Vegas Blackout key collection is the "room key"-- I have noticed in the last couple of days that I am getting some of the keys for this specific collection from OTHER rooms, I got multiple copies of the Joker key & the Vault key from the Pirate's Paradise room. So my strategy (for now) is to ramble around in the rooms I have unlocked so far & see what I can find.
I have searched through a lot of guides and help topics, but no one seems to have found a definite answer on how to get all the keys, will post an update if I make any more progress. Good Luck!!