ATC VECTORS AN ACR MDT INTO A CONFLICT WITH AN SMT.

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I WAS INBOUND TO PIA VOR ON THE 075 DEG RADIAL (HEADED WSW); AT 6000 FT. PIA APCH CALLED OUT TFC NW OF MY POS 'SBOUND IS AN MDT.' I SIGHTED THE TFC AND RPTED; 'TFC IN SIGHT.' PIA APCH REPLIED; 'MAINTAIN VISUAL SEPARATION; THE ACFT AT 8000 FT WILL BE DSNDING THROUGH YOUR ALT.' THE MDT CALLED PIA APCH AND SAID THAT HE DID 'NOT HAVE THE TFC (MEANING ME) IN SIGHT; BUT WE'VE GOT HIM ON TCASII.' IF THE COURSES OF EACH ACFT WOULD HAVE REMAINED CONSTANT; THE MDT SHOULD HAVE PASSED IN FRONT OF ME AS IT FLEW THROUGH MY ALT. AS THE MDT NEARED MY POS; PIA APCH INSTRUCTED THE MDT TO 'TURN L HDG 170 DEGS.' THE MDT COMPLIED AND ALSO RESPONDED THAT HE STILL DID NOT HAVE ME IN VISUAL CONTACT. ON THIS NEW HDG; COLLISION WITH THE MDT WAS IMMINENT. AS I PREPARED TO MANEUVER MY ACFT; PIA APCH INSTRUCTED THE MDT TO 'TURN L HDG 150 DEGS FOR TFC.' THE MDT COMPLIED AND COMMENTED AGAIN THAT HE STILL DID NOT HAVE ME IN SIGHT. OUR ACFT PASSED WITHIN 1 1/2 MI HORIZLY AT EQUAL ALTS. I WAS PUT IN A SIT WHERE I WAS EXPECTED TO AVOID AN ACFT THAT WAS MUCH FASTER THAN ME; WITHIN A FEW MI OF MY POS; AND MANEUVERING AND DSNDING UPON ME; ALL WITHOUT HAVING ME IN VISUAL CONTACT. PIA APCH HAD GIVEN ME THE RESPONSIBILITY OF MANEUVERING TO AVOID THE DSNDING MDT; BUT I WAS FACED WITH NOT KNOWING HOW TO MANEUVER CONSIDERING THE FOLLOWING 2 FACTS: 1) PIA APCH HAD BEGUN TO GIVE THE MDT VECTORS TOWARD MY ACFT; AND I HAD NO IDEA WHICH WAY HE WAS GOING TO TURN THE MDT NEXT. 2) THE MDT HAD ACKNOWLEDGED THAT THEY WERE TRACKING MY ACFT ON TCASII; AND I DID NOT KNOW IF THE MDT WOULD BEGIN EVASIVE MANEUVERING DUE TO TCASII COMMANDS; SINCE WE WERE IN SUCH CLOSE PROX. AS A COMMON PRACTICE; I HAVE SEEN CTLRS ASK PLTS TO MAINTAIN VISUAL SEPARATION FROM OTHER ACFT TO ALLOW A SMOOTHER OR FASTER FLOW OF TFC; ESPECIALLY WHEN OPERATING IN A BUSY AREA. I DO NOT KNOW WHAT THE PIA APCH HAD IN MIND. TFC DID NOT SEEM TO BE ESPECIALLY BUSY AT THIS TIME. IT APPEARS TO ME THAT THE PIA APCH CTLR MERELY 'BAILED OUT' OF HIS RESPONSIBILITY TO MAINTAIN SEPARATION BTWN IFR TFC BY INSTRUCTING ME TO 'MAINTAIN VISUAL SEPARATION;' THEN PROCEEDING TO RECKLESSLY DSND THE MDT. IT ALSO APPEARS THAT HE SAW THE CONFLICT TOO LATE AND TRIED TO CORRECT IT WHEN HE TOLD THE MDT TO 'TURN L HDG 150 DEGS FOR TFC.' I BELIEVE IT WAS A VERY DANGEROUS SIT THAT COULD HAVE BEEN AVOIDED; HAD THE CTLR HANDLED IT DIFFERENTLY. ANOTHER 60 SECONDS FOR THE MDT AT 8000 FT WOULD HAVE CERTAINLY PROVIDED BETTER SEPARATION. WAITING FOR ONE OF THE MDT CREW TO OBTAIN VISUAL CONTACT OF MY ACFT WOULD HAVE BEEN SAFER ALSO. IT WAS NEARLY IMPOSSIBLE FOR ME TO MAINTAIN A SAFE SEPARATION BTWN THE 2 ACFT SINCE THE MDT WAS FASTER THAN ME; DSNDING; AND NOT MAINTAINING A CONSTANT HDG. I THOUGHT THE INTENTIONS OF PIA APCH WERE FOR ME TO MAINTAIN SEPARATION WITH THE MDT AS HE DSNDED THROUGH MY ALT SBOUND; BUT THIS EVIDENTLY WAS NOT THE CASE. I WAS UNAWARE OF THE INTENTIONS OF BOTH THE CTLR AND THE MDT.