HDG TRACK DEV IN A NIGHT OP.

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STUDENT AND I WERE RETURNING TO DAY FOLLOWING A NIGHT INSTRUCTIONAL FLT. DAY APCH HAD ASSIGNED US A VECTOR OF 100 DEGS WHILE WE WERE APPROX 7 MI N OF THE ARPT; EXPECTING THE VISUAL APCH TO RWY 24L. APCH EXPLAINED THAT THE VECTOR WAS FOR SPACING TFC. SHORTLY THEREAFTER; THE CTLR ALERTED US TO ADDITIONAL TFC APCHING FROM OUR 5 O'CLOCK FOR RWY 18. SHORTLY AFTER WE CROSSED THE FINAL TO RWY 18; I CALLED THE TFC AT 5 O'CLOCK IN SIGHT. MY STUDENT MISTAKENLY THOUGHT THAT THIS TFC WAS THE CAUSE OF THE INITIAL 100 DEG VECTOR. SO; HE BEGAN A R TURN TO APPROX 160 DEGS AT WHICH TIME I TOLD HIM TO RETURN TO THE 100 DEG HDG. NO SOONER HAD HE BEGUN TO BANK L WHEN ATC CALLED TO ASK WHAT HDG WE WERE ON. I TOOK THE CTLS AND BROUGHT US HARD L TO 100 DEGS NOTIFYING DAY APCH AS WE PASSED THROUGH 110 DEGS. THE CTLR DID NOT SEEM UPSET; SHE JUST WANTED TO KEEP HER TFC SEPARATED! I MADE A POINT TO TELL MY STUDENT THE HAZARDS OF CHANGING COURSE/HDG AND COMPLYING WITH ATC INSTRUCTIONS IN REF TO SPECIFIC TFC IN A HVY TFC ENVIRONMENT. THE OPPORTUNITY FOR MISIDENT IS ABUNDANT.