1. Any modification to the default theme such as what you describe can definitely be done with some customization. We would recommend reading
this documentation if you are planning to modify our themes.
2. AppThemes does not officially support third party plugins, which means we cannot guarantee that every plugin will definitely be compatible with our themes.
3. Again, this is something that could technically be done, but going back to answer #2, if the plugin doesn't work with Vantage then you could be looking at a lot of customization work, not only with making changes to the theme layout, but to actual functions in order to get the plugin working as well. Thanks :)