FLC OF ACR LTT ACFT OPERATING UNDER VFR ENTERED IMC DUE TO CONFLICTING TFC HOLDING ABOVE AND BELOW.

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BEFORE DEP FROM LGA OUR RELEASE WAS AMENDED TO VFR DUE TO THE WX BEING SATISFACTORY FOR VFR FLT. AFTER DEP WE WERE GIVEN A CLB TO OUR REQUESTED ALT OF 6500 FT. UPON LEVEL OFF AT 6500 FT WE WERE ADVISED THAT THE NEXT CTLR (THE LAST NY SECTOR BEFORE MCGUIRE APCH) WOULD NOT ACCEPT THE HDOF AND THAT WE MUST CLB OR DSND FROM OUR REQUESTED ALT. WE WERE ABOUT TO LEAVE THE NY TCA; BUT TO ACCOMMODATE THE CTLRS I SAID I WOULD CLB TO 8500 FT. THERE WERE A FEW SCATTERED CLOUDS AT LOWER ALTS THUS MY DECISION TO CLB. AT THIS TIME I WAS PROCEEDING DIRECT RBV VOR AND WAS INFORMED THAT THERE WAS HOLDING IN PROGRESS OVER THE VOR. I WAS IN VISUAL CONTACT WITH 2 ACFT HOLDING OVER RBV; ONE 500 FT ABOVE US (9000 FT) AND ONE 500 FT BELOW US (8000 FT). THESE WERE JET ACFT AND FLYING RATHER LARGE RACETRACK PATTERNS IN VMC. I ELECTED NOT TO CLB TO 9500 FT DUE TO CLBING THROUGH THE 9000 FT JET'S ALT; OPPOSITE VFR TFC; AND LIMITED CLB PERFORMANCE. (OP ABOVE 10000 FT IS NOT PERMITTED BY OUR OPS MANUAL FOR NONPRESSURIZED ACFT.) AS WE APCHED RBV VOR; I REALIZED THAT THERE WERE SCATTERED CLOUDS OVER THE VOR; THUS I ELECTED ALSO NOT TO DSND. WE WERE ADVISED THAT THE JETS MAY BE DSNDING SOON. AS A NOTE; WE WERE IN CONSTANT RADAR CONTACT; AND RADIO CONTACT UP TO THIS POINT. RADIO CONTACT WAS THEN TERMINATED AS WE LEFT THE TCA AND THE NEXT SECTOR WOULD NOT ACCEPT US. WE CONTINUED OVER RBV VOR; STILL IN VISUAL CONTACT WITH THE JETS WHEN THE TCASII ISSUED A MONITOR VERT SPD AND A DSCNT RATE OF GREATER THAN 500 FPM WAS NOT PERMITTED. A FEW SECONDS LATER ANOTHER TCASII RA WAS ISSUED AND NO CLB WAS PERMITTED. LEVEL FLT OR A DSCNT OF UP TO 500 FPM WERE THE ONLY 2 OPTIONS AS WE PASSED OVER THE VOR. CLOUDS OVER THE VOR COULD NOT BE AVOIDED BY THE STANDARD MINIMUMS UNDER VFR IN CTLED AIRSPACE. WE MERELY SKIMMED THE TOPS OF THE CLOUDS BUT NEVER ENTERED AT 8500 FT. AS WE CONTINUED OVER THE VOR IT WAS OBVIOUS FROM OUR TCASII READOUT THAT THE JET ABOVE US WAS ISSUED A DSCNT. THIS IS WHY THE JET PASSED WITHIN 300 FT ABOVE AS NOTED ON OUR TCASII. THE JET BELOW US PASSED 500 FT BELOW ALSO AS PER THE TCASII. I HAVE NO IDEA WHY THE JET ABOVE US WAS ISSUED A DSCNT WHEN OUR ALT AND RTE WAS KNOWN BY ATC. ATC FAILED TO PROVIDE THE IFR JET WITH ADEQUATE SEPARATION. THE CONSEQUENCE WAS A CLOSE ENCOUNTER WITH THE JET AND THE INABILITY TO MAINTAIN CLOUD AVOIDANCE REQUIRED UNDER PART 91 RULES.