The publisher attempts to market the book and seeks reviews in appropriate media. The author is required to cooperate in the marketing effort. The big publishers all work from this model. This may, in the long run, prove to be a poor business plan, but it seems to be the current mode of business. However, this would only be part of an author's effort to make the manuscript marketable.
Many authors work with a literary agent to find a publisher.
There are various scams associated with literary agents. Be especially sure to determine if the literary agent has made a reasonable number of sales of similar manuscripts during the past 12 months. Again, it is highly recommended that you review all publishing related contracts with a competent lawyer specializing in intellectual properties. More likely, the smooth salesperson will, without making any real promises, lead the author to believe that the vanity publisher will make considerable effort to market the title. Reviewers and bookstores know who the vanity press scammers are and will have little, if anything, to do with them.