ACR ON NIGHT VISUAL TFC PATTERN FOLLOWS WRONG TFC AND HAS CONFLICT WITH ANOTHER ACFT.

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DURING NIGHT VMC TO SLL; WE WERE BEING VECTORED ON A DOWNWIND TO RWY 34L TO FOLLOW AN ACR Y (A REGIONAL JET). FOLLOWING AN ACKNOWLEDGEMENT FROM US THAT WE HAD THE TFC IN SIGHT; WE WERE CLRED FOR A VISUAL APCH TO RWY 34L. WE WAITED UNTIL THE TFC WE THOUGHT WE WERE FOLLOWING MADE ITS BASE AND TURN TO FINAL BEFORE BEGINNING OUR BASE TURN FROM DOWNWIND. PASSING THROUGH 7500 FT MSL DURING OUR TURN TO FINAL; WE RECEIVED A TCASII RA TO CLB; FOLLOWED SHORTLY BY ATC INSTRUCTIONS TO CLB TO 8000 FT MSL AND FLY 340 DEGS. WE WERE INFORMED THAT WE HAD TURNED INSIDE THE REGIONAL JET TFC AND CAUSED A CONFLICT. THE OVERRIDING CAUSE WAS ACCEPTING A VISUAL AT NIGHT ON AN ACFT THAT COULD NOT BE POSITIVELY IDENTED. WE WERE NOT AWARE THAT THE TFC WE THOUGHT WE WERE FOLLOWING WAS THE WRONG ACFT (AN ACFT Z; MD80). ALL LIGHTS LOOK THE SAME AT NIGHT AND DISTANCE DISCRIMINATION IS NOT POSSIBLE. SUPPLEMENTAL INFO FROM ACN 449004: AT NIGHT; DURING VMC; CONTINUE UNTIL ON AN INTERCEPT HDG TO FINAL. DO NOT REFER TO THE PLANE BY ACFT TYPE AT NIGHT. THE ONLY THING THAT CAN BE SEEN IN SOME LIGHT. ACFT LIGHTS DO NOT IDENT AN ACFT. ALSO; WHEN ISSUING A VISUAL CLRNC TO FOLLOW ANOTHER PLANE; GIVE THE POS OF THAT PLANE TO HELP ELIMINATE MISTAKEN IDENTITIES.