WHILE BEING VECTORED FOR AN ILS APCH; RPTR'S ACFT (X) CAME WITHIN CLOSE PROX OF ANOTHER ACFT (Y) THAT WAS ALSO BEING VECTORED BY DAB TRACON. THE SECOND ACFT (Y) WAS BEING WORKED BY A DIFFERENT CTLR ON ANOTHER FREQ. THIS RESULTED IN AN NMAC OPERROR.

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I WAS THE INSTRUCTING CFII ON AN INST (IFR FLT PLAN) XCOUNTRY FLT DAB-MLB-DAB. ON THE WAY BACK TO DAB; WE REQUESTED ILS RWY 7L TO DAB. WHILE OUR ACFT WAS BEING VECTORED DOWNWIND (S SIDE OF LOC) MY STUDENT FLEW; AS ASSIGNED BY ATC; WESTERLY HDG. I DO NOT RECALL EXACTLY WHAT THE ASSIGNED HDG WAS. THE ALT; I BELIEVE; WAS ASSIGNED AS EITHER 1600 FT MSL OR 2000 FT MSL. I DO KNOW; BEYOND ANY DOUBT; HOWEVER; THERE WAS NO DEV ON OUR PART FROM ASSIGNED ALT AND HDG. SUDDENLY; I SAW AN ACFT AT OUR 2-3 O'CLOCK POS. HE SUDDENLY TURNED SHARPLY TOWARDS US (HE MADE A L TURN). INITIALLY (BEFORE THE ABRUPT MANEUVER) HE SEEMED TO BE FLYING PARALLEL TO US. I WAS MOMENTARILY BLINDED BY OTHER ACFT'S HEADLIGHT. HE MISSED US BY APPROX 200 FT. I ESTIMATE; FROM THE MOMENT I SAW THE OTHER ACFT TO THE NEAR MISHAP; IT WAS NO MORE THAN 30 SECONDS; POSSIBLY EVEN SHORTER. OUR ACFT WAS WELL LIT. WE USED NAV LIGHTS; BEACON; STROBES AND 2 LNDG/TAXI LIGHTS. I WAS NEVER INFORMED OR NOTIFIED BY ATC ABOUT THE PRESENCE OR PROX OF OTHER ACFT. FURTHERMORE; I SCANNED FOR TFC. IMMEDIATELY AFTER THE OCCURRENCE; I RPTED THE EVENT TO THE CTLR; WHO IMMEDIATELY EXPLAINED THAT THE OTHER ACFT'S PLT MUST HAVE GOTTEN CONFUSED. I DO NOT RECALL ANY CONVERSATION BTWN THE PLT OF THE OTHER ACFT OR ANY APCH CTLR. I BELIEVED HE COULD HAVE BEEN ON THE N FREQ. AS I MENTIONED EARLIER; OUR FLT WAS AN IFR INSTRUCTIONAL FLT DURING WHICH I PAID PARTICULAR ATTN TO MY STUDENT'S STRICT ADHERENCE OF ATC INSTRUCTIONS. WE DID NOT DEV FROM ASSIGNED ALT; HDG OR ANY OTHER INSTRUCTION ISSUED BY ATC. I WAS NEVER PROVIDED WITH ADEQUATE IFR SEPARATION FROM ANOTHER ACFT; WHICH I LEARNED LATER WAS ALSO ON IFR FLT PLAN. WHEN I WAS VECTORED ON FINAL; LATER ON DURING THE SAME APCH; THE CTLR WAS APOLOGETIC AND EXPRESSED WHAT SEEMED TO ME AS GENERAL STATE OF CONFUSION EXPERIENCED BY ATC AND POSSIBLY OTHER ACFT. THE FOLLOWING DAY; I WAS CALLED BY THE TWR REPRESENTATIVE AND ASKED FOR DETAILED DESCRIPTION. THE CTL TWR MGR INFORMED ME THAT ATC MADE AN ERROR IN VECTORING ON THE NIGHT OF THE INCIDENT.