“Big City Comedy.” Vestron. $59.95. The good news about this 55-minute tape: It features John Candy, Billy Crystal, Martin Mull and Fred Willard--some of the most subtly creative comedians of the ‘80s. But there’s more bad news than good: This is a merely passable TV special from 1980 with the uneven flavor of the disappointing early-'80s syndicated show Candy made during a break from SCTV. Some unknown and unimpressive comedians also take part. Crystal (whose “current” role in “Soap” is brought up) and the others, though some of their low-key sketches are funny, were operating at half-ability (half-wit?). Information: (203) 968-0000.