If they didn't use vanity names, then people would only remember the first/last few random characters and the phishing scheme could very well still work, just it'd be less readable for visitors. I don't think we can assume that if all the characters were random they would remember them all better.
I don't think people would remember them better if completely random. Rather, I think if they're completely random, people might correctly assume they can't, and remain appropriately skeptical; if they include a vanity prefix, people seem likely to remember the vanity prefix and somewhat less likely to pay attention to the rest.
