All of our themes require their own instance of WordPress to run, so they are not designed to all be used on a single installation. The way around this however, is that you can install each theme in their own subdirectory e.g.
yourdomain.com <== Your Vantage installation
yourdomain.com/jobs <== Your JobRoller installation
yourdomain.com/classifieds <== Your ClassiPress installation
In order to allow users a single login across all installations, your best option would be to look at a
WordPress multisite setup.
If you are looking at modifications to any of the themes or installation services, you can find developers for hire on our
Certified partners page. If you make contact directly with an outline of your requirements, they will be able to provide a quote for the work. Thanks.