It's amazing to use the phrase "watch Xmas Vacation now" but yeah it really seems to have survived hasn't it? Weirdly the European and Vegas ones have been forgotten by comparison.
I don't think I've ever actually sat through the entire original Vacation film from beginning to end--I think I've just pieced it together from seeing in various hotel rooms and cable viewings and such.
At a friend's house they were watching some National Lampoon's movie and one of the previews said "From National Lampoon, the people behind Animal House! Vacation! And 30 other films you don't care about!" or something like that. I guess it doesn't have any reason to exist as a brand name anymore *except* to bring up the ideal of those two films.