Really I don’t know why it’s no possible to check the theme from the page ‘Appearance > Customize’. I’m going to find the problem and try to resolve it.
Anyway, with the plugin WPBakery Visual Composer it is not the perfect place to modify the pages. The best way is from the edition of the pages, the option ‘Fronted editor’. And I don’t know if the plugin is supported from the ‘Customize’ page.