A fictitious girl that a teenager (or occasionally college student) will claim that they hooked up with while on recess.
Vacation hookups are usually invented by kids towards the bottom of the social heirarchy, who have no success with girls in real life and are insecure about this. By providing what they consider a plausible tale of sexual virility that is difficult to disprove, they hope to garner respect and make themselves more popular.
Unfortunately vacation hookups never usually achieve their intended goal, as such stories are usually met with a lot of scepticism if the guy in question has never shown any ability to score with women anywhere else.
"Stuart claims he lost his virginity in Mexico during spring break, but it sounds like a vacation hookup to me."