Each theme requires their own instance of WordPress to run, so as mentioned above, you would need to look at installing each in a subdirectory
e.g.
yourdomain.com <== Your Vantage installation
yourdomain.com/jobs < Your JobRoller installation
The best solution for allowing a single login across multiple themes is to set up a
WordPress multisite network. You should be able to find a number of existing threads in the forums on this topic if you wish to know more. Thanks.