Vince Vaughn is the clown prince of frat-pack frivolity. Paul Giamatti is the master of modern misanthropy. Put them in a movie about the sibling rivalry between Kriss Kringle and his black-sheep brother, and naturally you'd expect something more Bad Santa than The Santa Clause. But against-type casting is about as edgy as Fred Claus gets. ''It was conceived as a family movie and it is a family movie,'' says producer Joel Silver of the decidedly PG film. ''It's funny, but it's also heartwarming and emotional.'' Sanitizing his usual persona, Vaughn plays the title character, who moves back home to the North Pole from Chicago after a fight with his girlfriend (Rachel Weisz). Soon he's corrupting the elves (played by Ludacris, John Michael Higgins, and a circus troupe of Russian little people) and stressing out his hyper-responsible brother. Wedding Crashers director David Dobkin who recruited Vaughn to the project says he wanted to tell ''a very deep story of acceptance and forgiveness.'' But staying on the nice side of things proved tricky for the cast during shooting last year in England's Shepperton Studios, where mad improvising was the rule. ''The natural inclination was to get a little naughty,'' says Giamatti. ''You know, turning a ho-ho-ho into a ho-ho-whore. They let me say 's---' one time, but I'm sure it won't make it.'' Sounds like someone's getting coal in his stocking this year.