You can definitely share the user database between multiple WordPress installations..there are many tutorials on how to do this if you search Google, such as
this one.
AppThemes themes are designed as stand alone WordPress themes, so they do require their own instance of WordPress to run. So, in saying this, if you wanted to have a website using both JobRoller and Vantage you can easily achieve this by installing the themes in a subdirectory. e.g.
yourdomain.com <== Your Vantage installation
yourdomain.com/jobs <==Your JobRoller installation
Then use the WordPress custom menus function to create links in the navigation menu for each. You can of course then make use of the tutorial as mentioned above to share the user databases, requiring only one logon between both installs.
It's just BuddyPress we can't be sure of as it's a plugin, and therefore compatibility issues may come into play when using it with one of our themes.