Is the movie "Vacation" (2015) a sequel, reboot, or remake?

From an NPR interview:

[Vacation's] Co-writers and co-directors John Francis Daley and Jonathan Goldstein say the R-rated movie is not a reboot or a remake, but very much a sequel.

"We've been very clear from the get-go, even with the studio, that we had no interest in trying to remake the movie that National Lampoon made in the '80s 'cause it's just so good that we wouldn't wanna tamper with it. It just works," Jonathan Goldstein tells NPR's Arun Rath. "And so we felt it was organic to bring it to the next generation."

I think it's also safe to say what it isn't:

[Reboot:] In serial fiction, to reboot means to discard all continuity in an established series in order to recreate its characters, timeline and backstory from the beginning.

The movie Vacation does not discard all previous continuity. It is not a reboot. A remake's definition is a bit looser (it's basically any reimagining of a source material), but this is not a shot-for-shot remake. It contains a few callbacks to the original film (like the red Ferrari scene) but also different characters, scenarios, locations and dialogue. So if the creators say that this is a sequel, I'll take their word for it.

As a matter of terminology, Vacation is a sequel. The terms you use can be easily defined to differentiate them:

• Sequel A film/show dealing with, or connected to the events of the primary film/show. Continuity between the two are intact. A Prequel falls under this, continuity wise.

• Remake A film retelling the same story as the previous version. Slight changes, update to time period, new actors. Continuity is not intact between them, but nothing is really changed, as the same story is told.

• Reboot A film using the same characters, but most often not the same story. All continuity is discarded. Single movie reboots do exist (Fantastic Four, Superman Returns).

• Spin Off A film that takes a single thread or character of the primary film, and develops its own, unconnected plot.

Vacation not only directly references the events of National Lampoon's Vacation, the entire plot is put into motion by it.

First of all, we're talking here about National Lampoon's franchise. So all questions about sequel/remake/reboot categorization are kinda moot by definition (but well-answered by others).

National Lampoon was a humor magazine than spawned dozens or films. They are related - sometimes by characters, sometimes by location and sometimes just by a sense of humor - but it's rather loose relationship. Some, of course, are more tightly knit together (eg telling the saga of Griswold family), but categorizing them as sequels or spinoffs is very difficult. Just because relationships between them vary so much, you're forced into arbitrarily deciding between remake and reboot based on continuity inconsistencies - when creators never really cared about continuity.

This particular example (Vacation 2015) is actually 5th(!) in National Lampoon Vacation sub-series. And it was originally indeed planned as a remake, but eventually during it's development it was turned into a proper sequel instead (hence Chevy Chase reprising Clark). So, not even creators of one film could really make up their minds, only adding into confusion.

You also need to take into account that reboots and remakes are very fashionable recently, so some people might incorrectly report sequels as re***s, just to make it sound more cool.