The Vacation movies have always hinged on one single element: Clark Griswold. He's the affable, flawed, frustrated everyman who, for every action he takes that we agree with, makes an equally ill-advised move, resulting in comedy. Sweet, sweet comedy.
Ed Helms is not that man. His cheery chipmunk-cheek demeanor and general obliviousness makes him a shitty hinge. When we watch the world shit on Clark, we want to sit down and have a drink with him, then hit on younger women. When we watch the world shit on Ed Helms, we just want to punch him in the face even more.
This will have a poor script and even poorer casting choices. Boo.