B757 ACFT CLRED FOR APCH; PF; FO COMMENCED DSCNT TO CLRED ALT USING VERT SPD. WHEN 'ALT CAPTURE' ANNUNCIATED ON ADI; THE CAPT RESET THE ALT ALERT TO 9000 FT; THE NEXT XING POINT. THE FO; CONTINUED DSCNT TO THE NEWLY SELECTED ALT; BUT PREMATURELY DUE TO ALT RESTRS ON THE APCH.

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INBOUND ON THE CIVET 1 ARR TO LAX AND HOLDING AT 12000 FT OVER ARNES INTXN. AFTER 2 TURNS IN THE HOLDING PATTERN; WE WERE CLRED FOR THE ILS RWY 25L APCH. TURNING INBOUND I (CAPT; PNF) RESET THE ALT ALERT TO 10000 FT. THE FO (PF) USED FMS VERT SPD MODE TO DSND TO 10000 FT. AFTER ALT CAPTURE ON ADI I RESET THE ALT ALERT TO THE NEXT XING ALT OF 9000 FT FOR SUZZI INTXN. THE FO WAS CONCERNED ABOUT GETTING HIGH ON THE APCH AND ENGAGED VERT SPD MODE TO START DOWN TO 9000 FT; NOT REALIZING HE WAS STILL 4 MI E OF ARNES INTXN (MINIMUM XING 10000 FT). APCH CTL STOPPED OUR DSCNT AT 9500 FT AND RECLRED US TO CROSS SUZZI INTXN AT 9000 FT; FUELR AT 8000 FT; AND THEN THE ILS RWY 25L APCH. I HAD MY HEAD DOWN; MOMENTARILY REVIEWING THE APCH PLATE; AND MISSED THE FO STARTING DOWN EARLY. THE USE OF VERT SPD MODE IS MORE COMMON ON THE B737-300 THAN IT IS ON THE B757. THE FO HAD TRANSITIONED FROM THE B737 A YR EARLIER AND PREFERRED USING THE VERT SPD MODE. THE USE OF THE VNAV MODE WOULD HAVE PREVENTED THIS MISTAKE. BUT EVEN MORE IMPORTANT IS SITUATIONAL AWARENESS ON THE PART OF THE FLC.