A B737 CREW; ON APCH TO SJC; COMMENCED DSCNT OUT OF THEIR ASSIGNED ALT BEFORE BEING 'ON A SEGMENT OF THE APCH;' AS ATC HAD INSTRUCTED.

Narrative

ALREADY BEHIND THE CURVE DUE TO A LATE DSCNT AT PAPEE ON THE EL NIDO 5 ARR. TOLD TO TURN AND INTERCEPT 118 DEG RADIAL OF SJC. HANDED OFF TO APCH WHO DSNDED US TO 5000 FT AND TOLD US TO GO TO GILRO; WHICH WAS ABEAM. THIS CAUSED SOME CONFUSION WHILE WE SORTED OUT WHERE WE WERE AS WE HAD EXTENDED FMS ONTO RADIAL. HE CANCELED THIS VECTOR; SAID NO PROB; AND TOLD US TO MAINTAIN 5000 FT UNTIL ON A SEGMENT OF THE APCH. THIS IS WHERE I MIGHT HAVE MESSED UP THINKING WE WERE ON A SEGMENT OUTSIDE OF FINOR. I BELIEVE THE SEGMENT DOES NOT BEGIN UNTIL FINOR. WE LEFT 5000 FT FOR 4300 FT; I BELIEVE 3-5 MI TOO EARLY; WHILE ESTABLISHED ON THE 118 DEG RADIAL. SEVERAL THINGS ADDED TO THIS. WE WERE USING THE VNAV PROC VFR FOR TRAINING. I WAS IN VOR MODE AS A BACK-UP. FO WAS IN NAV MODE. THE VECTOR TO GILRO THREW US OFF -- SIMILARLY TO THE CALI DISASTER -- AS IT WAS XA00 BODY TIME; BUT WHAT HAD ME WAY BEHIND WAS MY RECENCY OF EXPERIENCE HAVING FLOWN ONCE IN APRIL -- NOT AT ALL IN MARCH -- AND ONCE IN FEBRUARY. LOTS OF GOOD LESSONS HERE. THANKS FOR THIS PROGRAM. DID NOT DISCUSS INCIDENT WITH THE APCH CTLR.