In the crassly enjoyable Four Christmases, Reese Witherspoon and Vince Vaughn are forced to put in a yuletide visit to each of their divorced parents. It’s four home-for-the-holidays comedies in one, and the variety-pack structure works for the movie: Each nightmare household has a wacked flavor all its own, and none wears out its welcome. (The entire film runs 89 minutes.) The halfway humane joke is that Vaughn and Witherspoon have avoided marriage and kids, so what’s missing from their lives is the messiness of…life. Enter the coarse, bawdy corrective that is the Christmas family megaplex comedy. The quartet of house parties are sprinkled with witty actors like Jon Favreau, alarmingly convincing as Vaughn’s mohawked sociopath brother, and Mary Steenburgen, who gives Witherspoon’s pious mother a touch of erotic gumption. It’s a pleasure to see Vaughn play an actual character — a nervous motormouth who speaks in loop-the-loop sentences he uses to talk himself into things. Those sentences are the sound of an actor trying, in his way, to grow up. B?