MLG ON VECTOR FOR VISUAL APCH MADE UNAUTH DESCENT AND CREATED A LESS THAN STANDARD SEPARATION OCCURRENCE WITH ANOTHER ACFT. APCH CTLR SAW POTENTIAL CONFLICT AND APPLIED VISUAL SEPARATION AFTER MLG REQUESTED IT.

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COPLT FLYING; IFR LAX TO ONT. WHILE ON V21 (SLI-PDZ) APCHING PDZ AT 9000'; CTLR ASKED IF WE WANTED A VISUAL APCH. I REPLIED THAT WE DID. HE SAID (TO THE BEST OF MY RECOLLECTION); 'LEFT HDG 040 FOR VISUAL APCH; RPT ARPT IN SIGHT.' AS WE WERE NEARLY ABEAM THE ARPT AT THIS TIME; THE F/O ASKED ME TO ALERT THE CABIN TEAM OF LNDG (THIS IS A NEW PROC ON SHORT LEGS TO COMPENSATE FOR THEIR NO LONGER HAVING THE NO SMOKE SIGN AS AN ALERT). I ALSO TRIED TO CALL OUR COMPANY TO GET A GATE ASSIGNMENT (AS REQUIRED BY COMPANY POLICY). AS I SWITCHED BACK TO APCH; COPLT CALLED FOR FLAPS 1. I NOTICED WE WERE DSNDING; LOOKED AT THE ALT SELECT WINDOW AND SAW THAT THE F/O HAD SET 4500'. I THOUGHT ALL WAS WELL UNTIL CTLR SAID TO CLB IMMEDIATELY TO 8000'. I SAW TFC AT 12 O'CLOCK ABOUT 2 MI AND SAID WE COULD MAINTAIN VISUAL SEP. CTLR SAID TO MAINTAIN VISUAL SEP AND 7000'. HE SAID WE SHOULD STILL BE AT 9000' ASSIGNED. THE COPLT HAD THOUGHT WE WERE CLRED FOR A VISUAL APCH AND I HAD ACCEPTED HIS SELECTION OF 4500' IN THE ALT WINDOW AS A CLRNC W/O CONFIRMING IT WITH HIM. I SHOULD HAVE CONFIRMED REASON FOR DSCNT WITH F/O. BEING SO CLOSE TO THE ARPT IT SEEMED TO BE A LOGICAL THING TO BE DOING; SO I ACCEPTED IT. THERE SHOULD BE NO PROC WHICH TAKES 1 PLT ON A 2-M CREW OFF OF ATC FREQ AFTER LEAVING HIGH LEVEL CRUISE ALT; AS IT LEAVES NO BACK UP. WHILE CTLR ACTED PROPERLY AND DEVIATION WAS CREW ERROR; A CONFIRMATION TO 'MAINTAIN 9000'' AFTER 'HDG 040 DEGS FOR VISUAL APCH; RPT ARPT IN SIGHT;' WOULD HAVE CLRED THE CONFUSION THAT THE F/O HAD.