ACR X TCASII RA WITH VFR Y EVASIVE ACTION TAKEN. SEE AND AVOID CONCEPT.

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RECEIVING RADAR VECTORS. HAVING BEEN ON A SE HDG WE WERE TOLD TO TURN R TO 230 DEGS AND WERE IN AN ASSIGNED DSCNT TO 8000 FT. AS WE STARTED THE TURN AND WERE THROUGH 10000 FT WE GOT A TA ON A TARGET TO OUR R 800 FT BELOW AND CLBING. THE CAPT SWITCHED TO 5 NM RANGE AS I SCANNED OUTSIDE. WITH THE TARGET STILL CLBING WE GOT AN RA TO DSND. WE STOPPED OUR TURN WHICH WAS TAKING US INTO THE TARGET AND BEGINNING THE PUSHOVER WHEN THE RA CHANGED TO A CLB. THE CAPT; WHO WAS FLYING; IMMEDIATELY EXECUTED THE CLB MANEUVERS. AT ABOUT THIS SAME TIME I SPOTTED THE ACFT Y AND VISUALLY ENSURED THAT WE WERE ON A SATISFACTORY EVASIVE TRACT. WE GOT THE 'CLR OF CONFLICT' ADVISORY WHEN WE BROKE THROUGH 10000 FT. WE COMPLETED THE ROUNDOUT AT 10300 FT AND REINITIATED OUR DSCNT TO 8000 FT AND TURN TO 230 DEGS. IT TURNED OUT THE TARGET WAS A TURBOPROP Y TWIN WHO I BELIEVE WAS VFR CLBING TO 9500 FT TO RELEASE JUMPERS. HE WAS ON THE FREQ AND I BELIEVE IN RADAR CONTACT. WHEN WE ADVISED ATC OF THE CONFLICT A DIFFERENT CTLR CAME ON THE RADIO WITH A VAGUE ACKNOWLEDGEMENT. THIS IS THE THIRD RA I'VE HAD WHILE BEING RADAR VECTORED IN 1 MONTH. ATC ONLY BEING RESPONSIBLE FOR SEPARATION BTWN IFR TFC IS ABSURD. BEFORE TCASII; DID WE JUST NOT KNOW HOW MANY CLOSE CALLS WE HAD?