After following the right dialogue tree, the player will be able to sell Vault jumpsuits to Sarah. She will accept jumpsuits from any of the in-game vaults, except for suits stolen from the guest rooms in Vault 21. With a Barter check of 65, she will accept Boomer jumpsuits and Boomer flightsuits. Vault 3 suits can also be bought at Mick & Ralph's, but they will only have 1-4 suits, and once you've purchased them they do not restock.
You will receive caps, Strip reputation, and a discount on the Vault 21 guest room. After you bring her 30 Vault suits, she'll mention that she's missed you when you visit her next. You will then be able to have sex with her as you both "catch up" with one another in your room, which gives the Well Rested bonus (which is normally not earned by sleeping in Vault 21's guest room). Also, after two Speech checks, 65 and 75 respectively, she will agree to have sex after receiving suits, along with accepting Boomer suits.
There is no limit to how many times you can sleep with Sarah for the Well Rested bonus, but in hardcore mode, it does not lower the sleep meter, and simply passes time. This makes it necessary to rest normally in one of the beds, which also passes time, leading to dehydration and starvation. For this reason, sleeping with her simply for the bonus in hardcore mode is very inefficient. It's better just to find a bed that addresses both SLP and well rested.
Although you can convince Sarah to accept Boomer clothing, giving her these items will not count towards the total number of vault jumpsuits you must give her in order to have sex with her.
You may need a high Perception (8+) to start the conversation that leads to you asking her about about the suits.
If your character is female, the two speech checks will not show up, even after you've completed the quest. You will also not be required to rent a room prior to having sex with Sarah. The game will simply recognize that you've completed the quest and the option to have sex with her will be automatic.
Every time you give her a Vault Jumpsuit, you gain Strip fame.
pcxbox360ps3 Sarah won't accept the Vault 21 jumpsuit you are given at the beginning of the game. [verified]
xbox360ps3 Sarah will sometimes use a deeper female voice upon greeting you or after finishing a trade. [verified]
xbox360 Sarah will sometimes take Vault 24 suits, but won't accept Vault 22 suits. [verified]
pc If you take jumpsuits from your companion while you're in the same room as she is, she does not detect it in your inventory. Fixed by taking the jumpsuits outside the store then reentering. [verified]
xbox360 If the player leaves the vault after giving Sarah the 30 jumpsuits and she agrees to sex, without first having sex with Sarah, she will remain in the gift shop and it becomes impossible to interact with her. [verified]
pc Sarah will pay the courier for vault suits in companion inventories, as well as paying for suits the courier drops on the floor (!), but will be unable to actually remove any suits not in the courier's inventory. Showing up with 30 suits and dropping 29 on the floor, picking them up and dropping 28 etc will allow the player to be paid for 465 suits (a little time consuming, but as described, would increase the reward 15-fold). [verified]
pcxbox360ps3 With Veronica as a companion Sarah will come to the room to have sex but then leave before and the player will be stuck. [verified]