At age 38, Reginald VelJohnson has found his calling: playing affable men in blue. After years as a classically trained New York stage actor who'd ranged from Staggerlee to Shakespeare, VelJohnson got his big break three years ago when he landed the role of Al Powell, the levelheaded, Twinkie-loving sergeant who talked Bruce Willis through the original Die Hard (he reprised the role in Die Hard 2). That led to his casting as Carl Winslow, Chicago policeman and beleaguered patriarch of ABC's Family Matters, where he has to share scenes with a houseful of child actors and play straight man to America's favorite nerd, Jaleel White's whining Steve Urkel. ''For a while I found myself overplaying, trying to outdo the kids,'' says VelJohnson, who grew up with four siblings in a middle-class family in Queens, N.Y., ''but I've learned to play it down a little bit.'' Now he's living in a Hollywood condo (newly remodeled, with a TV in every room), getting yells of ''Hi, Carl'' in shopping malls, and shooting next season's Grass Roots, an NBC miniseries with Corbin Bernsen, in which he plays big change of pace here an ex-cop.