Other mistake: In the scene at the beginning, when the robbers take Harold's wallet, the robber who had the gun says "American express card" but, when he says "don't leave home without it" the subtitles say the goon on the left says it, even though you can see the goon on the right move his mouth. (01:04:30)
Answer: He could have looked up her address in the phone book. Just before Bruce arrives at Vikki's apartment, Joker asks Bob the Goon for - a phone book. Also he could have been tailing Bruce and found her apartment that way, after all he does show up not long after Bruce does.
Alan Keddie