E145 HEARS AND EXECUTES INTERCEPT TO RWY 19L ILS AT IAD. APCH CTLR INTENDED RWY 19R AND RECLEARS APPROPRIATELY.

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ON THE ARR INTO IAD; WE WERE TOLD TO EXPECT RWY 19L BY APCH CTL. AFTER BEING TURNED OFF THE BARRIN 1 ARR FOR A BASE VECTOR; WE WERE TOLD TO FLY HEADING 210 AND INTERCEPT THE LOC. BOTH MYSELF AND THE CAPT HEARD RWY 19L AS OUR ASSIGNED RWY. THE READBACK OF OUR CLRNC TO INTERCEPT THE LOC WAS FOR RWY 19L AS WELL. AFTER TURNING TO INTERCEPT THE RWY 19L LOC; THE APCH CTLR TOLD US TO MAKE AN IMMEDIATE R TURN BACK TO 210 DEGS AND INTERCEPT THE RWY 19R LOC. WE COMPLIED; DESCENDED INTO VISUAL CONDITIONS; CALLED THE ARPT IN SIGHT; AND CONTINUED THE VISUAL APCH TO LAND ON RWY 19R. EITHER THERE WAS NOT CLOSED LOOP COM BETWEEN US AND ATC; OR ATC MISTAKENLY PUT US ON THE WRONG APCH. WE BELIEVE THAT WE HEARD AND READ BACK RWY 19L EACH TIME. DURING THIS TIME; ATC HAD; UNDER POSITIVE CTL; VECTORED A VFR ACFT TO PASS WITHIN 500 FT OF US WITHOUT NOTIFYING US UNTIL WE WERE WITHIN A COUPLE MILES AT THE MOST. ATC ALSO HAD AT LEAST ONE OTHER NON-AIRLINE ARR THAT WAS MAKING SOME EXTRA REQUESTS AND TAKING UP RADIO TIME. IT COULD HAVE BEEN EASY FOR ATC TO MISS OUR READBACK OR ASSIGN US THE WRONG RWY; OR FOR US TO MISS A LAST MINUTE RWY CHANGE.