Each of the products are stand alone themes, so they are not designed to be used all on one WordPress installation as they each require their own instance of WordPress to run.
What you could do however, is consider installing the various themes on their own subdirectory such as this:
e.g.
www.yourdomain.com/ <== Your Vantage installation (or other)
www.yourdomain.com/classifieds <== Your ClassiPress installation
www.yourdomain.com/jobs <== Your JobRoller installation
To take it further, these could all be set up on a
multisite network in order to allow one login across all the installations. With some styling to make all the themes "match", you would have the appearance of one unified website.