Question: Were Katherine and Sebastian's parents even still together? And where were they? I'm guessing the movie was set in the summer and they were wealthy, so had taken off for a long holiday.
Answer: We the audience are never told one way or the other where the parents are. But according to Katherine, Sebastian's father is diddling the maid (or something). Meanwhile the mother, either one, is either, a) dead, b) in rehab or c) as you say on a permanent vacation.
They are together. Kathryn says that "the parental units called." They're in Bali, and the maid is probably a hotel maid there.
Answer: Kathryn tells Sebastian that "the parental units called," as if they are together at the place that they called from. He asks if her "gold-digging" mother is enjoying Bali. She jokes about his "alcoholic" father "diddling the maid." I think the parents called from a hotel in Bali (so she is referring to a hotel maid). They are together, even if the marriage is just a sham at this point.