Making vanilla use your existing user table is not that easy, actually. Making your site use vanilla's user table is easier. At this point that kind of integration would require programming knowledge. Do you have any?
I've changed the repository with a new method of integrating with external applications and posted a blog entry about it. Vanilla 1 will be able to more easily integrate with other applications as a result... http://lussumo.com/swell/42/Data-Objects--External-Application-Integration/