ACR MLG; ON ILS APCH TO BUR; CAME VERY CLOSE TO SMA WHILE OVER VNY ARPT.

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WE WERE HANDED OFF TO BUR APCH ABOUT 30 NM OUT AND 10000'. AS I REMEMBER ATIS WAS SAYING X6F. AT OUR POS IT WAS MORE LIKE 2 NM VISIBILITY. EVERYTHING PROCEEDED NORMALLY UNTIL ABOUT 4800' MSL WHEN I NOTICED AN SMA AT ABOUT 11:30-12 O'CLOCK AND SAME ALT; AND APPROX 1 MI. AS WE WERE DSNDING AT 1000' FPM IT WAS APPARENT TO ME THAT THERE WOULD NOT BE A PROB AS WE WOULD PASS SLIGHTLY UNDER (300') AND TO THE W OF THE TFC. I QUESTIONED APCH ON THIS AND THEY SAID THEY DID NOT SEE ANYTHING ON THEIR RADAR. ABOUT 15-30 SECS LATER APCH CALLED WITH 'TFC AT 1 O'CLOCK AND 1-2 MI; PROBABLY OUT OF THE PATTERN AT VNY.' AS THE F/O WAS FLYING I IMMEDIATELY SEARCHED FOR THE TFC AND A FEW MOMENTS LATER AN SMA (BLUE AND WHITE; I THINK) PASSED DIRECTLY UNDER US AT ABOUT 2800' MSL. NO EVASIVE ACTION WAS TAKEN BECAUSE BY THAT TIME IT WAS TOO LATE. WE FINISHED THE APCH; MADE A NORMAL LNDG. AFTER WE BLOCKED IN I CALLED TWR TO GET THE NUMBER OF APCH CONTROL; BUT AFTER TALKING TO THEM (TWR) AND UNDERSTANDING THE SITUATION AT VNY I FELT IT WAS NOT NECESSARY. ALSO; I SHOULD MENTION THAT ALTHOUGH ATIS WAS SAYING VISIBILITY WAS 6F; THE VISIBILITY WAS PROBABLY ONLY 1 1/2-2 NM AT THE FIELD AS WE PICKED UP THE RWY AT ABOUT THAT DISTANCE. AS I UNDERSTAND IT; ACFT OPERATING FROM ANOTHER FIELD IN A CTL ZONE DO NOT HAVE TO BE IN CONTACT WITH ATC AT THE PRIMARY ARPT. IN THIS CASE AND PROBABLY OTHERS; THE POTENTIAL FOR DISASTER LOOMS VERY LARGE. I BELIEVE SOME CHANGES SHOULD BE MADE TO AT LEAST KEEP TFC FROM THE APCH PATH WHERE HIGHER SPD ACFT ARE OPERATING AND WHERE APCHS ARE KNOWN TO BE IN PROGRESS.