After reading this thread and comment on John Scalzi's blog (prompted by the Harlequin decision to create a branded vanity press), I sort of vaguely understand the difference now, between a self-publishing firm and a vanity press.
From the point of view of the person writing the book and forking over the cash, the difference is the amount of $$ forked over (substantially less for the self-publishing side of things) and the amount of grandiose-sounding promises (substantially more for the vanity press folks). If the book actually sells to people, all of the net profits go to the writer who self-publishes, but substantially less than 100% goes to the vanity press writer.
From the point of view of someone being told about all of the books someone has written, however, they look pretty equal. This became an issue some years ago, when a pompous idiot made demands of me on the basis of his important books...all of which were self-published. Yes, I understand the value of self-publishing...but I also understand that there is no automatic editorial or vetting process.
Am I missing something here?