A B767-300 HAS AN NMAC WITH A CESSNA TYPE ACFT 5 MI N OF PDX WHILE ON A VECTOR FOR LNDG RWY 10R.

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WHILE DSNDING FROM 10000 FT TO 5000 FT ON THE DOWNWIND TO RWY 10R; 5 MI N OF PDX; WE WERE ADVISED TO LOOK FOR VFR TFC AT 2 O'CLOCK; 5 MI. PASSING THROUGH 6800 FT WE GOT AN RA TO CLB. I DISCONNECTED THE AUTOPLT AND CLBED TO 7300 FT. AS WE BOTTOMED OUT AT 6600 FT WE SAW A CESSNA CLOSE ABOARD ON THE R SIDE; APPROX 400 FT BELOW US. THE CESSNA WAS ON A SBOUND HDG. SINCE WE WERE ON A 275 DEG VECTOR WE WOULD HAVE CROSSED DIRECTLY IN FRONT OF THE CESSNA AT ABOUT 400 FT IF WE HAD CONTINUED THE DSCNT. PORTLAND ARPT SHOULD BE A CLASS B AIRSPACE. PLUS THE CTLR SHOULD NEVER HAVE DSNDED US THROUGH THE CESSNA'S ALT. THE CTLR KNEW THE CESSNA'S ALT TO BE 6500 FT. SUPPLEMENTAL INFO FROM ACN 430989: WE COULD NOT MAKE VISUAL CONTACT WITH THE TFC; BUT WATCHED IT APCH ON A CONSTANT BEARING ON TCASII. TCASII SHOWED THE ACFT AT 6300 FT MSL. PASSING 7000 FT MSL WITH NO VISUAL; WE SLOWED THE DSCNT; AND THE TCASII ACTIVATED AT 6700 'CLB; CLB' WITH AN RA WARNING. WE BOTTOMED OUT AT 6500 FT MSL AND CLBED BACK UP TO 7300 FT MSL. LEVELING AT 7300 FT MSL WE RPTED TO APCH; WHO IMMEDIATELY TURNED US ON BASE (BACK TOWARD THE TFC) AND A DSCNT TO 3000 FT MSL. APCH DID NOT RESPOND TO OUR QUESTION ABOUT THE PROX OF THE VFR TFC.