I like to use the plugin “Strong Testimonials” to handle testimonials, since it have a good user interface.
However, it is also nice to use the style that comes with the Ticketbox theme.

So, for those who think the same, here’s a solution.

1: Use a child theme of Ticketbox theme, and upload this folder to the child theme folder: https://www.dropbox.com/s/pw923yk7pvgutj4/strong-testimonials.zip?dl=0

2: Install & activate Strong Testimonials, and go to Views->Add New.
Chose Mode: Display
Scroll down and chose Style: Template: Child Theme: StrongTicket

3: Copy the generated shortcode, which will look something like this: [testimonial_view id=2].

4: Go to Visual Composer->Shortcode Mapper. Click the button “Map Shortcode” and follow the instuctions.

Done! The “Strong Testimonials” plugin is now integrated with the theme, and can be selected in the Visual Composer.