A B737-300 FO; PNF; COMPLAINED THAT MIA TRACON AND FLL TWR SEEMED COMPLACENT WHEN MAINTAINING SEPARATION FROM VFR ACFT; WHICH CAUSED 2 DIFFERENT TCASII RA ALERTS.

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LEVEL AT 6000 FT WITH MIA APCH INTO FLL; POINTED OUT APCHING ACFT LEVEL AT 6000 FT. WE ADVISED THAT ACFT WAS NOT IN SIGHT AND THAT WE WERE LEVEL RIGHT ABOVE THE RPTED OVCST. A MIN LATER; THE ACFT TURNED INTO A TA ADVISORY AS ATC ASKED IF WE HAD TFC IN SIGHT. WE RESPONDED NEGATIVE; HE MIGHT BE IN THE CLOUDS. ATC RESPONDED WITH A TFC ALERT AND TOLD US TO DSND TO 4000 FT. AT THAT TIME OUR TCASII RA ADVISORY COMMANDED A DSCNT. WE STARTED A RAPID DSCNT INTO IFR CONDITIONS. WE NEVER DID SEE THE ACFT AND MISSED IT. I ASKED APCH IF THIS ACFT WAS ON AN IFR OR VFR FLT PLAN; AND ATC RESPONDED THAT HE WAS ON A VFR FLT PLAN AND NO RADIO CONTACT. NOT EVEN 5 MINS LATER ON FINAL APCH RWY 27R ILS INTO FLL AT 1000 FT; WE HAD AN ADDITIONAL RA FROM AN ACFT VFR LNDG ON RWY 27L. AGAIN; TWR THIS TIME ADVISED US ABOUT THE APCHING ACFT; BUT TOLD US HE WOULD CROSS OUR FINAL BEHIND US. NOT SO; HE CROSSED OUR FINAL APCH AT 500 FT -- SO CLOSE THAT WE AGAIN RECEIVED AN RA 'REDUCE DSCNT.' I DO NOT KNOW WHAT TO SAY; BUT I FEEL THAT THIS RELAXED HANDLING OF VFR ACFT IS TROUBLING. WE LANDED SAFELY AT FLL; BUT SOME PAX MADE SOME REMARKS.