MD88 FLC DSNDED IN RESPONSE TO A TCASII RA CAUSED BY VFR PIPER CHEROKEE THAT INDICATED A LOWER ALT THAN RPTED.

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I WAS THE PF ON THE MD88 WHILE BEING VECTORED FOR THE ILS RWY 18L AT CVG. CINCINNATI APCH ADVISED ME AND MY CAPT THAT WE HAD CHEROKEE TFC AT 10-11 O'CLOCK; 6 MI LEVEL AT 8500 FT MSL. THE TFC APPEARED ON OUR TCASII AS A TA. APCH CTL THEN ADVISED US THAT THE CHEROKEE HAD OUR MD88 IN SIGHT. NEITHER MYSELF NOR MY CAPT HAD VISUAL CONTACT WITH THE TFC. AS THE TA MOVED CLOSER ON OUR TCASII; I RESTED MY THUMB ON THE AUTOPLT DISCONNECT SWITCH. OUR TFC APPEARED TO GET CLOSER HORIZLY AND VERTLY. EVENTUALLY; OUR TCASII SHOWED OUR SEPARATION AT 300 FT AND 2 NM. THIS CLOSE PROX CAUSED OUR TA TO BECOME AN RA. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND FOLLOWED THE RA'S INSTRUCTION TO 'DSND; DSND.' OUR EVASIVE MANEUVER CAUSED OUR ACFT TO LEAVE OUR ASSIGNED ALT OF 8000 FT AND DSND TO 7600 FT MSL. WE ADVISED THE APCH CTLR OF OUR ACTION AND THAT WE WOULD RESUME 8000 FT MSL SINCE WE WERE NOW CLR OF THE CONFLICT. THE CTLR SAID NOT TO WORRY AND ISSUED US INSTRUCTION TO DSND TO 4000 FT FOR CONTINUED VECTORS TO THE ILS RWY 18L. I CAN ONLY SPECULATE THE REASON FOR THE CONFLICT BEING CHEROKEE TFC HAVING AN INCORRECT ALTIMETER SETTING. SINCE THIS TFC HAD RPTED TO ATC THAT THEY HAD VISUAL CONTACT ON OUR MD88; I DON'T UNDERSTAND WHY THAT TFC WOULD CONTINUE TO REDUCE OUR SEPARATION TO THE POINT OF ACTIVATING OUR RA FOR EVASIVE ACTION.