A B737-300 FLC EXPERIENCES A LOSS OF SEPARATION DURING AN OVERSHOT TURN ONTO THE ILS FOR RWY 21L AT DTW; MI.

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WE WERE TURNING L FROM A 270 DEG HDG. AT 4000 FT ON A VECTOR TO FINAL RWY 21L. THE VECTOR WAS A POOR VECTOR BEING SO CLOSE TO THE INTERCEPT. APCH ASKED US IF WE SAW TFC AT 10 O'CLOCK POS OR THE ARPT. WE SAW NUMEROUS ACFT AND QUESTIONED HER ON 10 O'CLOCK POS OR 12 O'CLOCK POS. NO ARPT IN SIGHT DUE TO HAZE AND SUN SETTING. CTLR SAID 'FOLLOW THE DC9; CLRED FOR VISUAL RWY 21L.' WE WERE TURNING FOR RWY 21L AND GETTING CLOSER TO ACFT GOING TO R SIDE. WE THEN GOT A TCASII ALERT AND AN RA ALERT TO DSND. FO SAW ACFT AND SAID TURN R; WE WERE ALREADY DSNDING. ANOTHER CTLR CAME ON THE RADIO AND SAID CONTINUE TO 3000 FT AND GAVE US A VECTOR BACK TO RWY 21L. THE TWR CTLR ASKED US TO CALL TWR WHEN WE GOT ON THE GND. WE CALLED AND TALKED TO SUPVR. HE ASKED WHAT WE SAW AND THEN SAID THE CTLR SHOULD NOT HAVE HAD US ON A 90 DEG VECTOR TO FINAL AND THAT OUR RESPONSE TO THE RA WAS TECHNICALLY CORRECT. SUPVR SAID THIS WOULD BE AN INTERNAL (CTLR) SIT AND THAT THERE WOULD BE A QUALITY ASSURANCE AUDIT AND AS FAR AS HE WAS CONCERNED THE SIT WAS CLOSED. I THANKED HIM AND THAT WAS IT. I FEEL HAVING ACFT TO ALL PARALLEL RWYS AT THE SAME ALT AND TURNING IN ON EACH OTHER IS AN ACCIDENT WAITING TO HAPPEN. MAKING 80-90 DEG TURNS TO FINAL DURING SIMULTANEOUS APCHS ALSO CAN CAUSE RA'S AND ALSO THE ILLUSION TO THE PLTS THAT THEY ARE COMING TOGETHER; ESPECIALLY AT THE SAME ALT. SUPPLEMENTAL INFO FROM ACN 526589: I SAID I HAD THE ACFT AT 10 O'CLOCK POS AND ANOTHER AT 12 O'CLOCK POS. I THEN ASKED IF THE ACFT AT 12 O'CLOCK POS IS FOR THE OTHER RWY. THE CTLR CLRED US TO FOLLOW THE DC9 FOR THE VISUAL RWY 21L; THEN SAID THE OTHER ACFT IS FOR THE R SIDE. SUPVR(?) SAID IT WAS CTLR ERROR.