A DH8 FO RPT ON THE PENETRATION OF P56 AT DCA DURING A VECTORED ARR TO RWY 15.

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THE PROB AROSE WHILE BEING VECTORED FOR THE VISUAL APCH TO RWY 15 AT DCA. WE WERE ON R DOWNWIND FOR RWY 15 WITH THE ARPT IN SIGHT. DCA APCH ISSUED A HDG OF 050 DEGS FOR BASE LEG AND CLRED US FROM 6000 FT TO 4000 FT. WE WERE TURNED UNUSUALLY CLOSE TO THE ARPT FOR OUR ALT. IN ADDITION; APCH TOLD US OF TFC (VFR) AT OUR 11:30 POS AT 3500 FT. AS WE CONTINUED OUR DSCNT; APCH AGAIN GAVE THE VFR TARGET'S POS AND ALT. AS WE PASSED THE EXTENDED CTRLINE OF RWY 15; WE CALLED THE TFC IN SIGHT. AT THIS POINT WE WERE PROBABLY TOO HIGH FOR AN APCH TO RWY 15. THE CTLR ASKED IF WE COULD MAKE RWY 21. WE REPLIED YES. WE WERE CLRED FOR THE VISUAL TO RWY 21 AND ALSO TOLD TO MAINTAIN VISUAL SEPARATION WITH THE TFC (A HIGH WING CESSNA?). THE TFC BEGAN OR WAS IN A TURN. AS WE DSNDED THROUGH THE TFC'S ALT; IT WAS NECESSARY TO SHALLOW OUR TURN RADIUS TO AVOID THE TFC. AT THIS POINT; WE WERE AWARE OF OUR PROX TO AREA P56. HOWEVER; MOST OF OUR ATTN WAS ON AVOIDING THE BANKING VFR TARGET. WE KEPT OUR ACFT AT WHAT WE PERCEIVED TO BE A SAFE DISTANCE FROM THE TFC. I BELIEVE WE FLEW INTO P56 DURING THIS MANEUVERING AROUND THE TFC. FROM MY VANTAGE POINT I COULD NOT PHYSICALLY SEE THE EASTERN SHORE OF THE POTOMAC RIVER THAT DENOTES THE P56 BORDER IN THAT AREA. THE CAPT WAS INFORMED UPON LNDG (BY LANDLINE TO THE TWR) THAT WE HAD FLOWN INTO P56. THIS SIT COULD HAVE BEEN AVOIDED. I BELIEVE ANY ONE OF THE FOLLOWING WOULD HAVE PREVENTED FLT INTO P56: 1) ALLOWING GREATER SEPARATION FROM P56 IN APCH SECTORS. 2) KEEPING VFR TFC AWAY FROM APCH AREAS (TRANSIENT). 3) NOT ACCEPTING VISUAL APCHS; THUS NOT DSNDING. 4) NOT ACCEPTING DSCNT NEAR TFC OR NOT AGREEING TO MAINTAIN VISUAL SEPARATION. 5) IF A VECTOR IS GIVEN THAT PUTS THE ACFT ON A DIRECT COURSE FOR P56; ADVISE THE DISTANCE FROM P56 BORDER. 6) BETTER JUDGEMENT.