FLC OF A BEECH 1900 FAILED TO FOLLOW THE HEADING GIVEN BY ATC DUE TO A MISUNDERSTANDING OF VECTORS TO FINAL.

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ABOUT 120 NM FROM PHX ZAB GAVE US A VECTOR TO INTERCEPT THE PHX 015 DEG RADIAL INBOUND (FOSSL 3 ARR). THE VECTORS WERE ALL WESTERLY SINCE WE WERE ARRIVING FROM THE ENE. THE CTR CTLR GAVE US ABOUT 5 DIFFERENT CORRECTIONS; ALL DECREASING FROM APPROX 265 DEGS TO 240 DEGS IN 5 DEG INCREMENTS OVER A 80 NM DISTANCE. WHEN WE WERE 'HANDED OFF' TO PHX APCH CTL HE GAVE US A 220 DEG HDG FOR THE FINAL APCH COURSE. THE PNF READ BACK THE HEADING AND WE TURNED TO 220 DEGS. I DID NOT REALIZE THE VECTOR WAS A VECTOR FOR FINAL HOWEVER; AND A FEW MI LATER TURNED TO A HDG OF 195 DEGS AND INTERCEPTED THE 015 DEG RADIAL AS PREVIOUSLY INSTRUCTED. ABOUT 2 MINS LATER THE CTLR ASKED US OUR HEADING. THE PNF INFORMED ME THAT 220 DEGS WAS A VECTOR FOR FINAL AND NOT TO JOIN THE 015 DEG RADIAL. WE CORRECTED TO 220 DEGS IMMEDIATELY AND INFORMED THE CTLR OF OUR CORRECTION. THE CTLR ASKED US WHY WE HAD TURNED TO 195 DEGS. WE DID NOT RESPOND IMMEDIATELY AS THE FREQ STARTED TO GET CONGESTED AND THE CTLR GAVE US ANOTHER HEADING. THE REMAINING ARR WAS UNEVENTFUL. AFTER WE LANDED I TELEPHONED THE APCH CTL SUPVR TO TELL HIM WE TURNED BECAUSE WE INCORRECTLY THOUGHT WE WERE STILL INSTRUCTED TO INTERCEPT THE 015 DEG RADIAL DUE TO A MISCOM. THE SUPVR DID NOT KNOW ANYTHING MORE ABOUT IT AND THANKED ME FOR THE CALL. I NOW MAKE SURE THAT BOTH CREW MEMBERS UNDERSTAND NEW INSTRUCTIONS AND CLRNCS.