MD SUPER 80 FLC TOOK EVASIVE ACTION BY DSNDING AND TURNING IN RESPONSE TO A TCASII RA OF A KING AIR ACFT.

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WHILE BEING TURNED SW; OFF OF THE ARR ON VECTORS HEADED W AT 10000 FT; WE WERE GIVEN NOTICE OF VFR TFC CLBING SBOUND. I BELIEVE THE CTLR SAID 'YOU DO HAVE THAT TFC CLBING VFR AT 1 O'CLOCK POS; DON'T YOU?' ALMOST IMMEDIATELY WE HAD A TCASII ALERT CALLING 'TFC; TFC.' I LOOKED AT THE TCASII SCREEN TO SEE THE TFC INDEED CLBING TOWARD US. IT IMMEDIATELY TURNED FROM YELLOW TO RED AT WHICH TIME THE TCASII RA CALLED FOR US TO DSND. THE CAPT BEGAN A DSCNT AND I CALLED TO ATC 'ACR X DSNDING; RA.' I LOOKED AT TCASII TO SEE TO MY SURPRISE THAT THE TFC WAS CLBING THROUGH 9500 FT. THE CAPT BEGAN TO TURN WHILE DSNDING. TCASII CALLED 'INCREASE DSCNT RATE' AND ATC CALLED FOR US TO TURN R. I MISUNDERSTOOD THE HDG AS THE TCASII AURAL WARNING DROWNED OUT THE ATC VOICE COMMAND. ATC CORRECTED MY HDG READBACK; BUT THE CAPT HAD ALREADY HEARD THE CORRECT HDG; TOLD ME SO; AND WAS TURNING TO IT. WHILE IN A DSNDING R BANK; I LOOKED UP AND THROUGH THE CAPT'S WINDSCREEN AND 'DV' WINDOW SAW A KING AIR 90 WITH THE DOOR REMOVED PASS BY R TO L. MY ESTIMATE WAS 1/4 - 1/2 MI AND A FEW HUNDRED FT SEPARATION. AFTER CONFLICT RESOLUTION; THE CAPT BEGAN A CLB TO THE PREVIOUSLY ASSIGNED ALT. VERY SHORTLY THEREAFTER; ATC GAVE US VECTORS AND A DSCNT TOWARD SJC. ATC DID NOT INFORM US OF THE VFR TFC UNTIL IT WAS ALMOST TOO LATE. ATC ADVISED US ONLY ONCE OF THE TFC -- WHEN HE QUESTIONED US IF WE HAD THEM IN SIGHT. IT IS THE OPINION OF THE CAPT AND I THAT WITHOUT TCASII IT WOULD HAVE BEEN A MIDAIR COLLISION.