THE FLC WHILE ON VECTORS FOR AN APCH DSND BELOW MSA FOR THE AREA.

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AN INTERMITTENTLY INOP VHF XMITTER MAY HAVE CONTRIBUTED TO CONFUSION THAT RESULTED IN DSCNT BELOW MSA WHILE BEING RADAR VECTORED TO FINAL FOR THE VOR DME RWY 30R APCH AT SJC. I WAS THE PF IN VMC ON TOP OF SCATTERED TO BROKEN CLOUDS. BOTH OF US NOTED THE PROX TO TERRAIN; BUT BEING WELL CLR ABOVE IT; WE DID NOT IMMEDIATELY QUESTION OUR CLRNC. WE HAD JUST NOTICED THAT SOMEHOW THE ACTIVE COM HAD BEEN SWITCHED PREMATURELY TO TWR. WHETHER THAT WAS A RESULT OF SETTING OVER THE ACTIVE FREQ OR ACCEPTING ANOTHER ACFT'S FREQ; I'M NOT SURE; BUT BAY APCH FIGURED WHAT HAPPENED AND SOON SJC TWR ASKED US TO CLB TO 5000 FT. NEXT WE WERE GIVEN SEVERAL VECTORS; ONE OF WHICH WE RESPONDED TO WAS FOR AN ACFT WITH A SIMILAR CALL SIGN. THIS FACT COUPLED WITH AN INOP XMITTER MAY HAVE PREVENTED APCH CTL FROM HEARING OUR RESPONSE; THUS ELIMINATING THE FAIL SAFE FEATURE OF PRECISE READBACKS. THE APCH CONTINUED NORMALLY UNTIL WE WERE GIVEN A LOW ALT ALERT BY TWR. I HAD PREMATURELY TRANSITIONED TO VISUAL GUIDANCE AND FAILED TO MAINTAIN THE RECOMMENDED ALT AFTER ACQUIRING THE RWY. EQUIP FAILURE ASIDE; I BELIEVE COMPLACENCY PLAYED A ROLE IN THIS POORLY EXECUTED FLT. RECOGNITION OF THIS; PLUS IMPROVED CRM AND ADHERENCE TO NON PRECISION APCH PROCS; SHOULD HELP PREVENT A RECURRENCE OF THESE MISTAKES.