CTLR FAILED TO PROVIDE POSITIVE SEPARATION TO 2 ACR-SMT'S IN THE ARSA.

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THE INBOUND ACFT RPTED ON THE FREQ AT 11000' AND HAVING CURRENT ATIS INFO. THE ACFT WAS ON IFR FLT PLAN AND WAS DSNDED TO 7000'. WHEN THE ACFT WAS ABOUT 15 MI NW OF THE ARPT; (WHICH IS ABOUT 10 MI NW OF THE ROA VORTAC AS THE VORTAC IS ABOUT 5 MI NW OF THE ARPT). THE PLT RPTED THE ARPT IN SIGHT (WX WAS CLR; 50 MI VIS AT THE TIME). I ACKNOWLEDGED THE RPT AND TOLD PLT TO DSND TO 6000' AND LOOK FOR TFC DEPARTING RWY 24 VFR TURNING NBOUND AN SMT. PLT ACKNOWLEDGED LOOKING. SECOND ACFT; ALSO AN AIR TAXI; DEPARTED VFR TURNING NBND; RPTED ON FREQ. INBOUND ACFT WAS TURNED 60 DEGS RIGHT AND DSNDED TO 5000'. 45 SECS LATER TFC WAS CALLED AGAIN TO INBND; AND A TURN ANOTHER 20 DEGS TO RIGHT WAS ISSUED. OUTBND TFC RPTED THE INBOUND IN SIGHT APPROX 2 MI AWAY; INBOUND TURN HARD RIGHT AND DSNDED QUICKLY TO MISS OUTBND. THE ZDC DARC TRACK SHOWED THE INBOUND DID NOT TAKE THE INITIAL HDG AND DID NOT INITIATE TURN UNTIL THE SECOND HDG WAS ISSUED. ABOUT 40 SECS LATER. I FAILED TO ENSURE SEP BY USING VERT SEP; HOWEVER THE PLTS RELUCTANCE TO TURN AWAY FROM THE ARPT ON THE VECTOR ASSIGNED COMPOUNDED THE ERROR. ROA HAS AN ARSA AND TARGET RESOLUTION IS REQUIRED AND WAS NOT ENSURED. PERHAPS A LARGER AMOUNT OF HORIZ SEP IS NECESSARY BTWN IFR AND VFR ACFT IN AN ARSA. SUPPLEMENTAL INFO FROM ACN 88677: ABOUT 10 NM NW OF ROA APCH VECTORED US RIGHT TO 180 DEGS AND ADVISED US HE HAD SMT COMMUTER DEPARTING RWY 24 TURNING NBND. ACKNOWLEDGED WE WERE LOOKING FOR THE TFC. NEXT SMT COMMUTER CALLED US IN SIGHT. AT ABOUT 7 NM OF THE ARPT APCH VECTORED US RIGHT TO 200 DEGS AND CLRED US TO 5000'. INITIATED ABOUT A 500 FPM DSCNT; SPD 200 KTS; SLOWING AND TURNED RIGHT TO 200 DEGS AS I ROLLED THE WINGS LEVEL THE F/O AND I NOTED THAT WE WERE ON A HEAD ON COLLISION COURSE WITH THE SMT TFC AT A DISTANCE HORIZ OF APPROX 500'. THE F/O CALLED OUT UP OR DOWN! I FIRST ROLLED IN A SLIGHT RIGHT BANK; THEN FOLLOWED THROUGH WITH 20 DES NOSE DOWN AND 45 DEG BANK. THE SMT TFC; IN MY OPINION; DID NOT SEE US UNTIL WE MADE THE COLLISION AVOIDANCE MANEUVER MISSING HIM BY APPROX 150' VERTICALLY AND 200' HORIZ.