SMT PLT ENTERS VMC AND DSNDS VISUAL PRIOR TO NOTIFYING ATC. ALARMS SOUND AS MAKING CONTACT.

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SMX LOC DME (BACK COURSE) - A APCH TO RWY 30 SMX. BTWN KOAKS AND CAMCO ON THE APCH COURSE I ENTERED VFR CONDITIONS DURING THE DSCNT FROM 4600 AT KOAKS AND PRIOR TO THE 3000 REQUIRED FOR CAMCO AND I HAD THE ARPT IN SIGHT. I CONTINUED A VFR DSCNT TO LAND BUT DID NOT CALL THE TWR TO RPT 'VFR - ARPT IN SIGHT' QUICK ENOUGH. THE VFR DSCNT PATH I WAS THEN FLYING WENT BELOW THE PUBLISHED IFR STEP-DOWN APCH PRIOR TO REACHING CAMCO. THIS TRIGGERED THE ALARMS AT VANDENBERG APCH. THE SMX TWR CALLED TO RELAY A VANDENBERG CAUTION TO 'PULL-UP' AS I WAS TRYING TO CALL THE TWR TO RPT 'VFR.' WHEN I RESPONDED TO THE TWR WITH 'ARPT IN SIGHT' I WAS CLRED TO LAND BY THE SMX TWR. VANDENBERG APCH ASKED ME TO CALL THEM AFTER LNDG; WHICH I DID. THE TELEPHONE CONVERSATION WITH A VANDENBERG APCH CTLR CTRED ON WHAT TRIGGERED THEIR ALARMS. WHEN THEY LEARNED I WAS VFR AT THE TIME THEY THANKED ME FOR THE CALL AND IMPLIED NO FURTHER INTEREST. I BELIEVE THE PROCEDURAL MISTAKE I MADE WAS TO START THE VFR DSCNT TO THE LNDG BEFORE I NOTIFIED THE TWR THAT I WAS IN VFR CONDITIONS.