You could certainly use both themes on the same website, bearing in mind that each theme requires its own instance of WordPress to run.. so you'd install each in a subdirectory e.g.
yourdomain.com <== Your Vantage installation
yourdomain.com/classifieds <== Your ClassiPress installation
And if you're looking for users to have one login across both sites you'd want to look at a
WordPress multisite setup. This is generally how users who wish to use multiple themes on a single website will set things up. Thanks :)