Unlike Krupper, the Vault Dweller can speak with Flip even if not wearing robes like the Children of the Cathedral, although he will immediately attempt to escort the them to the Lieutenant. If the player character is wearing robes, he will tell them to go to the control room. Failure to comply will result in him attacking.
If the player character enters the cell that holds Sarah with Flip around, he will scream, "Don't you come near her! I'll kill you first!" showing it very clearly that Flip and Sarah have a romantic connection as a product of him watching over her. If Flip dies, it will greatly upset Sarah.