If you are looking to use two themes you can use them on the same site by installing them in subdirectories, e.g:
yourdomain.com <== Your Vantage Installation
yourdomain.com/classipress <== Your ClassiPress installation
In conjunction with the use of a
WordPress multisite setup.
There is also a child theme bundle available called
TwinPress (You still require purchases of Vantage and ClassiPress in order to use this), but it will give both themes a more unified look. This is designed also for a multisite setup. Thanks.