A B727 CREW INBOUND TO MEM DID NOT COMPLY WITH CTLR'S INSTRUCT TO FOLLOW B727 FOR RWY 18R.

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WE WERE ON VECTORS; COPLT FLYING; ON A 90 DEG INTERCEPT TO FINAL RWY 18R MEM. APCH ASKED US IF WE HAD OUR PRECEDING TFC IN SIGHT. WE SAID WE WERE UNSURE. HE SAID IT WAS THE 'SOUTHERN MOST B727.' WE SAID WE HAD THE TFC. HE TOLD US TO FOLLOW HIM. NEITHER THE COPLT NOR I HEARD HIM SAY EITHER WE WERE 'CLRED APCH' OR 'CLRED VISUAL.' WE BOTH WERE EXPECTING A 45 DEG DOGLEG TO FINAL TO INTERCEPT. AT A 90 DEG INTERCEPT WE SOON QUESTIONED OURSELVES AS TO WHETHER WE WERE CLRED AND DECIDED TO TURN TOWARD THE RWY. BY NOW; EVEN IN THE TURN; WE WERE ENCROACHING ONTO RWY 18L FIN AL APCH COURSE. AT THIS POINT; THE FINAL CTLR TOLD US TO TURN TO AN 'E HDG' TO DEPART THE PARALLEL APCH COURSE. WE WERE SUBSEQUENTLY VECTORED TO A RWY 18L LNDG. IN REVIEWING THE SIT AFTERWARDS; NEITHER THE COPLT NOR I FELT THE 'FOLLOW THAT B727' WAS CLRNC FOR THE APCH AND WERE EXPECTING A FURTHER TURN ONTO FINAL FOR RWY 18R. WHEN WE REALIZED IT WASN'T COMING; WE IMMEDIATELY BEGAN A TURN; BUT WERE THEN VECTORED ON AN EASTERLY HDG TO DECONFLICT WITH RWY 18L TFC. NEITHER THAT TFC NOR US HAD TO TAKE EVASIVE ACTION; BUT I FEEL IT WAS TOO CLOSE FOR COMFORT. NOT HEARING A 'CLRED FOR APCH' OR A 'CLRED VISUAL FOR WY 18R' CALL; WE DON'T BELIEVE A 'FOLLOW THAT B727' WAS CLRNC FOR US TO TURN ON COURSE AT THAT POINT.