Here are all the safehousese :
Spoiler:The following information was gathered from my Fallout: New Vegas Collector's Edition Strategy Guide. All credit goes to the author David S.J. Hodgson.
I will now list the homes, location and how to earn them. Please do not post until I am finished and say it is okay to do so. Thank you.
Before we begin; A note of warning: Sadly, there will be some small spoilers that are unavoidable.
Home: Comped Suite
Location: Atomic Wrangler (Casino/*****house in the town of Freeside. It's upstairs on the upper balcony.)
How to earn:
You must initiate and complete the "Debt Collector" quest given to you by Francine Garret, who is usually at the front counter during the day in the Atomic Wrangler. Do the first part of the quest, which requires you to collect debt from three different NPCs (You can go about this in numerous ways) and then do the second part, which requires you to either kill Caleb McCafferty or convince him to simply give you his hat in return for telling Francine that he's dead with a Speech Skill of 40. Return to Francine Garret afterwards and collect your reward, the key to the Comped Suite. (Alternatively, you can kill Caleb McCafferty wherever you see him and loot the key to his room from his corpse.)
Home: Presidential Suite
Location: Lucky 38 Casino
How to earn:
Well let's see... This what the guide says- "This swanky home becomes accessible only after Mr. House grants you access to it during the Main Quest, usually at the end of "Main Quest: You'll Know It When It Happens"."
Side note: Sorry, any more information than that and it'll spoil the Story/Main Quest. This would constitute a MAJOR spoiler rather than a small one. It appears we'll get it during the Main Quest somehow. Also, this home has a fair bit of upgrades available for purchase.
Home: The Tops Presidential Suite
Location: The Tops Casino
How to earn:
Only accessible by elevator with the Presidential Suite Key, which is given out during "Main Quest: Ring-A-Ding Ding" by Benny under certain circumstances.
Home: Ultra-Luxe Penthouse
Location: Ultra-Luxe Casino
How to earn:
Well let's see here, it says... "You must have an appropriate key (As part of the "Side-Quest: Beyond The Beef", or unlock the elevator manually." (It doesn't say what level of Lockpicking skill is needed.) Someone named Mortimer gives it to you, depending on what choices you make during the side-quest. I'm sure you can either kill him and loot the key from him or attempt to pickpocket it from him. Give it a try either ways.
Home: Guest Suite and/or Sarah's Room
Location: Vault 21
How to earn:
Guest Suite- You can rent it for one night at 20 caps a night or... Succeed at a speech challenge with a Speech Skill of 55 and negotiate a special price for a permanent rental agreement of 10 caps per night. Keep asking nicely about the vault and she'll give you a personal tour.
Sarah's Room- It says something about using "cunning" (Yes, I know what cunning is), but there isn't much information on that. You can also either pickpocket her Passcard to her room or murder her and loot it from her. Beware, as this will severely lower your reputation with the Strip.
Home: Brotherhood Of Steel Safehouse
Location: Brotherhood Of Steel Safehouse (Somewhere on the very northeast part of the map very close to the invisible boundary of the map. It's in rocky terrain near a small band of Fire Geckos. It's an unassuming shack enshrouded in a rock alcove.)
How to earn:
This shack can only be opened if you have the Brotherhood Of Steel Safehouse Key, awarded by the leader of the Brotherhood at the conclusion of "Side Quest: Still In The Dark".
Home: Dino Dee-Lite Rental Room
Location: Novac
How to earn: This is only accessible once you pay Jeannie May Crawford 100 caps and she gives you the Dino Dee-Lite Room Key. You can make yourself at home here and return as often as you like.
Home: Caesar's Legion Safehouse
Location: Caesar's Legion Safehouse (Directly in the south part of the map in the middle, all the way at the end. As with the Brotherhood Safehouse, this will be near the map boundary. It's near a small hill and a rock where Bighorners are grazing. The safehouse has a campfire outside.)
How to earn: You need to get a key that is usually awarded to you in and from a location called "The Fort".
Taken from a post at GameFaqs