A happily unmarried couple (Vince Vaughn and Reese Witherspoon) spend Dec. 25 with each of their four postnuclear families. Though the two stars twinkle and the parents (Robert Duvall, Sissy Spacek, Jon Voight, Mary Steenburgen) are all Oscar winners, the movie is a painful mix of farce, mush and propaganda for parenthood. Given this loser and Fred Claus, maybe Vaughn ought to swear off Christmas movies. 11/26