National Lampoon's Vacation is a classic series of comedy films, but why do two of them lack the famous branding in their title? The original National Lampoon's Vacation movie featured an embarrassing wealth of comic talent; it was written by The Breakclub Club's John Hughes (and based on a short story he penned for National Lampoon), directed by Harold Ramis and starred Chevy Chase at the height of his stardom. The result was a hilarious comedy that followed the Griswold family on a disastrous road trip, and the surprise success of the film soon led to a franchise.

https://screenrant.com/vacation-movies-national-lampoon-missing-why/