ZJX CTLR EXPERIENCED OPERROR AT FL250 AS FLT CREW MISTAKES HDG FOR ALT ASSIGNMENT AND CTLR FAILS TO HEAR AND RESPOND TO INCORRECT READBACK.

Narrative

WHILE WORKING THE ST JOHNS SECTOR AT ZJX; I ISSUED A CLRNC TO ACR X TO 'FLY HDG 330 VECTOR FOR YOUR CLB.' THE PLT RESPONDED 'UP TO 330 ASSIGNED.' INSTEAD OF TAKING THAT AS A VECTOR CLRNC; THE ACR TOOK THAT AS A CLB CLRNC. ACR X WAS THEN XFERRED COMS TO THE KEYSTONE SECTOR AND NEVER CHKED ON; INSTEAD JUST SITTING ON THE FREQ -- ALL THE WHILE IN A CLB TO FL330 IN THE SAME DIRECTION AS THE B737 IN VMC. KEYSTONE SECTOR ATTEMPTED TO HAVE THE ACR STOP HIS CLB OUT OF FL235; HOWEVER; BY THAT TIME THE ACR ACFT HAD ALREADY PASSED THROUGH FL240. THE CLOSEST THESE 2 PLANES GOT WITHIN EACH OTHER WAS 500 FT AND 2.3 MI. I UNDERSTAND AS THE CTLR THAT ISSUED THE VECTOR TO THE ACFT; I SHOULD HAVE CAUGHT THE BAD READBACK WITH UN-PRESCRIBED PHRASEOLOGY. I DO FEEL THAT THE ACR PLT SHOULD BE GIVEN A MAJOR PART OF THE BLAME HERE AS WELL: 1) HE CLRLY MISSED A CLR UNDERSTANDABLE CLRNC FOR THE VECTOR. 2) HE USED NO PRESCRIBED PHRASEOLOGY IN HIS READBACK OF THE CLRNC. 3) HE DID NOT CHK ON TO THE RELIEVING SECTOR. 4) AND THE MOST IMPORTANT REASON -- HE CLBED IN VMC AND YET FAILED TO SEE A B737 AT 500 FT AND 2 1/2 MI.