B737 ACFT IN DSCNT AND HEADING VECTORS OVERSHOT ASSIGNED ALT; BUT CORRECTED BACK. CTLR ISSUED VISUAL APCH.

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FLT ARRIVING IAD AT NIGHT IN VFR CONDITIONS ON A VECTOR TO A VISUAL APCH FOR RWY 1R; ARRIVING FROM THE NNW. BEING VECTORED FOR A RADAR L TFC PATTERN; 20-30 MI RADIUS FROM ARPT. FROM THE HVY RADIO TFC IT APPEARED ONLY 1 CTLR WORKING THE INITIAL AND FINAL APCH SECTORS AND THERE WERE ACFT TRAINING AT NEARBY SATELLITE UNCTLED ARPTS; AS WELL AS ARRIVING COMMUTER FLTS. OUR POS; 7-10 MI SSW OF IAD ON A ENE HEADING FOR RWY 1R. ACFT WAS BEING HAND FLOWN IN A VFR DSCNT AT 500-700 FPM; TO THE CLRED ALT OF 3000 FT. THE ARPT; RWYS 12 AND 1L COULD BE SEEN; BUT NOT RWY 1R. CTLR WANTED US TO RPT RWY 1R IN SIGHT FOR A VISUAL APCH CLRNC. WHEN WE COULD NOT RPT IT IN SIGHT THE CTLR VECTORED US TO A NNE HEADING. THE PF QUESTIONED THE HEADING WITH THE CREW TO VERIFY THAT THE ASSIGNED HEADING WAS WHAT WAS HEARD; BECAUSE THE ASSIGNED HEADING TOOK THE ACFT AWAY FROM THE FINAL APCH COURSE FOR RWY 1R WITHOUT THE RWY IN SIGHT. THE ACFT POS NOW WAS ABOUT 3-5 MI FROM ARPT; 1/2 MI TO THE W OF THE EXTENDED CTRLINE OF RWY 1L HEADING NNE. PNF RESPONSE WAS THAT THIS VECTOR WAS GIVEN TO ALLOW SEPARATION WITH A COMMUTER ACFT ON A R BASE TO FOLLOW US TO RWY 1R. PF FELT ACFT WAS BECOMING HIGH AND CLOSE-IN FOR A VISUAL APCH TO RWY 1R. CHKED ALT OF ACFT AND ACFT WAS AT 2800 FT DSNDING. PF STARTED TO ARREST THE DSCNT AND QUESTIONED THE PNF ABOUT THE CLRED ALT. THE RESPONSE WAS 3000 FT LAST REMEMBERED ALT ASSIGNED AND A CLB STARTED BACK TO 3000 FT. ABOUT THIS TIME VISUAL APCH CLRNC WAS GIVEN AND THE FLT RESUMED AN ENE HEADING TO INTERCEPT THE RWY 1R FINAL APCH COURSE. DSCNT RE- ESTABLISHED WITH NORMAL APCH AND LNDG AND ARR. IN FUTURE; RECOMMEND THE PF USE AUTOPLT TILL ESTABLISHED ON FINAL APCH COURSE IN ORDER TO FREE UP MENTAL WORKLOAD TO BETTER IMPROVE SITUATIONAL AWARENESS. ALSO USE LOWEST APCH SPD ATC WILL APPROVE UNTIL CLRED FOR THE APCH AND ESTABLISHED ON FINAL.