Matt Borlenghi and Liz Vassey have a lot in common. The two star in UPN's Pig Sty; he as a lawyer, she as his building's superintendent. And earlier in their careers both were underutilized by All My Children. Borlenghi, 27, who played AMC's humorless hunk Brian, gripes, ''On soaps, they write for the women.'' Vassey, 22, who did her share of brooding as dowdy teen wife Emily Ann, admits, ''It's a relief to not cry anymore.'' Each has also recently appeared on ER: Vassey played a nympho (''It's fun being recognized on the street for that,'' she deadpans), and Borlenghi, a young man about to be married who is diagnosed with AIDS. Besides a renewal for Pig Sty, they both dream of guest shots on Seinfeld. Borlenghi even has a part in mind: ''I've always seen myself as Kramer's younger brother.''