Hi Jen, the themes aren't designed to be used on a single installation as they require their own instance of WordPress to run, so essentially you'd need to look at installing Vantage in a subdirectory in order to use them together. Once styled, they can appear as if they are one single website.
e.g.
yourdomain.com <==Your ClassiPress installation
yourdomain.com/directory <== Your Vantage installation
And if you want user logins to link between themes then a good option would be to use a
WordPress multisite setup.
Ratings cannot be turned off via the Vantage admin dashboard, so this would require some theme modification if you wanted to do this. All categories and sub-categories are set up by you, and the "Popular Categories" widget will display categories and subcategories.
We're pleased to hear you are happy with the ClassiPress theme, and your ClassiPress site looks very nice by the way :)