To clarify, Vantage is a stand alone theme, not a plugin, so no you would not be able to use another theme at the same time. Vantage isn't designed to separate listings into locations as you have described, but you could achieve something similar through use of a
WordPress multisite setup (which would essentially require multiple instances for each location, or if you set up the categories as the locations for a niche market it could be similar to
this site from our
customer showcase. These options would of course likely require additional customization on your part for location selection etc.
The widget plugin is created by one of our marketplace developers, but not AppThemes, so for best clarification of its functionality we would recommend you connect with the developer directly by posting on the comments tab of that marketplace page and they can respond.
To your further notes, you can have a look at the customer showcase for examples of what AppThemes customers are doing with the theme. What you mention would require customization, however as our themes are not encrypted, you are free to make modifications if you wish. Thanks.