There is no encryption which would prevent you from making modifications to the theme layout if you wish. In doing so, we would recommend the use of a
child theme so that you don't lose all of your changes in the process of an upgrade. AppThemes does not officially support third party products, so we unfortunately cannot guarantee that all plugins will be compatible with our themes. You could create a price field using the custom forms functionality, but you would likely want to modify the layout further if it's specifically for products. There is no eCommerce functionality built into the theme though, so by default you wouldn't be able to sell items online via the theme.
Add-ons and child themes for Vantage can be found in the
AppThemes marketplace, and there are some
recommendations for plugins here which you could use for signing in by other methods. Thanks.