I think a lot of the times titles may change because distributors feel a title may not make sense to the country they are being distributed in, so change titles to something more friendly to that countries vocabulary.
Back in the day i don't think we had summer camps for kids in the U.K, except Butlins, caravan parks or scout"s etc, so Nightmare Vacation makes more sense.
Although "Butlin's - Hi di die campers" may have worked.
...Something like that.
Other reasons used to be Grindhouse distributors would sometimes try to re sell an older film as a new release, (sometimes adding in new scenes) back in the day when video and the internet didn't exist.
For this reason some films have 4 or so different titles.
Also Italian films used to shamelessly rip off other films so in their own countries they were released as Zombie 2, Alien 2 etc, but released them in the originating country as something else i'm guessing to not attract attention to the fact they were blatant rip offs.
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MIKE: I've got it! Peter Cushing! We've got to drive a stake through his heart!
VYVYAN: Great! I'll get the car!
NEIL: I'll get a cushion.