ACR X TCASII RA WITH VFR ACFT REFUSED CLRNC RADAR VECTOR HDG.

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ACR X DOWNWIND AT IAD FOR RWY 1L (5000 FT MSL/190 DEGS/170 KTS). OUR TCASII DISPLAYED TFC AT 10 O'CLOCK/300 FT BELOW OUR ALT (PREVIOUSLY ISSUED BY IAD APCH; WHICH THEY RPTED AT 4500 MSL FT/VFR). WE VISUALLY ACQUIRED TFC IN THAT LOCATION ON A WESTERLY HDG. ATC DIRECTED L TURN TO 090 DEGS FOR BASE LEG. NEARLY SIMULTANEOUSLY WITH RADIO CALL BY APCH TO TURN TO 090 DEG; WE GOT A TCASII CLB RA. WE TOLD ATC OF TFC CONFLICT; THAT WE COULDN'T TURN; AND THAT WE WERE RECEIVING AN RA. ATC AGAIN INSTRUCTED THE 090 DEG HDG AND SAID THE TFC WAS NO FACTOR SINCE IT WAS AT 4500 FT. WE TOLD ATC WE WOULD TURN AFTER AVOIDING THE TFC. TURN WAS DELAYED APPROX 1-2 MI. ATC ASKED US TO CALL THEM AFTER LNDG. THE ATC TRACON SUPVR FELT WE SHOULD HAVE TURNED AS INSTRUCTED; RATHER THAN DELAYING THE TURN TO AVOID WHAT WE BELIEVED TO BE A POTENTIAL NEAR MISS. SUPPLEMENTAL INFO FROM ACN 297481: ATC INSTRUCTED US TO FLY A 090 DEG HDG. AT THIS TIME WE ALSO RECEIVED A CLB RA. I TOLD ATC THAT WE HAD TFC TO OUR L AND THAT WE WERE RECEIVING AN RA. ATC AGAIN INSTRUCTED US TO FLY A 090 DEG HDG AND THAT THE VFR TFC WAS NO FACTOR BECAUSE IT WAS AT 4500 FT. I AGAIN TOLD ATC I WOULD TURN THE ACFT ONLY AFTER I WAS CLR OF THE VFR TFC. I DELAYED THE TURN APPROX 1-2 MI.