Simplest way to define a "destination wedding" is "some place I would NOT be going to at all if it weren't for the happy couple having decided to get married there".
Doesn't matter if it is Hawaii, Disney-XXXXX, a castle in Scotland, a historic church in (anyplace, really), or the side of a mountain - if it is NOT where I would pick to go at that time - then it is a "destination wedding".
I might decide to go to Hawaii, Disney-XXXXX, or the other places on a vacation - but I would be in charge of when I went, what I did, and what I might wear while doing it. It's doing it purely & simply as a wedding "guest/audience" that makes it a destination wedding.
Because while I'm at the wedding, reception, breakfast, shower, whatever else is going on - I am not "having my vacation", I am resting up for the next wedding event. Or after the last wedding event before going home...,