First, we have his pledging his allegiance to magical thinking by making the ridiculous assumption that going to Not!Disneyland will make his family wonderful because it does in the ads. He assumes that his rather ordinary, happy family is falling all to bits because it doesn't look like Daddy Knows Best and his cure for the alleged illness is also based on pure wind.
This leads us to his being entranced by the siren song of impracticality. Just as John assumes that he can mollify Elly (and keep her from spending him into the county poorhouse) by getting her a new toaster when it's clear that they've needed to upgrade the kitchen since the strip began, Clark refuses sight unseen to fly to Los Angeles because airplanes mean the supposed isolation he wishes to fight. It should be noted at this juncture that when Rusty and Audrey get in the horribly impractical car that he ends up defending because the alternative is admitting to being a craven sucker, they're wearing Walkmans because Clark is talking out of his own asshole like always.
The true test of his (lack of) character comes in when hardship arises. He has to deal with an aunt who hates him (and who dies on him), a dog he accidentally kills, people who don't give a shit who some manure-for-brains dick from Chicago in an ugly-ass car is when they penalize him for being a dumbass and a family that's getting pissed off with him. Instead of packing it in like a sane, sensible person, our boy redoubles his efforts in order to avoid the humiliation of admitting that he's going the extra mile to achieve piss-all. When the park is closed and he should realize that he screwed his family over for less than nothing, instead of an admission of guilt or remorse, he 'makes it up to them' by holding a security guard hostage after delivering a profane rant blaming the park's owners for his own folly and ignorance.
The worst part of the film is the downer ending. Mrs Griswold doesn't divorce the man's ass, the park's owner feels sorry for him and he's allowed to go right on being a thoughtless mutant manchild because He Means Well. If that don't make him a Pattersaint, nothing will. He's rocking the total lack of accountability for being a douchecanoe patriarch John gets to enjoy. Also, he learns nothing from anything because he's Chevy Chase and we're not.