It's really hard to make a book a best seller, but the most important things are that it has to have a really broad appeal, and it has to be published well, with full sales and marketing support, and a good bookshop presence. I'm afraid that as your mother's book was published by a vanity press it won't get these things; and because it's already been published, she will really struggle to find a good publisher willing to take it on now.
In your mother's case I'd consider self publishing it once I was sure I'd got my rights reverted by the publisher; and perhaps contacting any organisations which offer support to people affected by the condition she's written about. It won't make the book reach best-seller status, but it might allow more people to be helped by it, which would be a good thing.