Vantage is a stand alone theme, so it is not designed to be used like a plugin as you describe. In order to use it with an existing website, you can easily install it in a subdirectory and simply add it to the menu of your existing site (potentially with restyling of the theme in order to "match" your existing theme).
e.g.
yourdomain.com <== Your existing homepage/theme
yourdomain.com/directory <== Your Vantage installation
You can find information about
AppThemes API's here. Technically it's possible to do whatever you like given the relevant coding knowledge to implement it, as there is no encryption which would prevent you from doing so. AppThemes doesn't officially support customizations or offer these services, however if you're looking to hire a developer to provide more information and a quote about whether or not they can do this you can find them on our
Certified Partners page. Thanks.