Andrea P answers:
wordpress 4.5 and following versions have introduced updated versions of the packaged js/jquery libraries.
therefore a lot of themes and plugins had issues and their author have been forced to relase patch to fix the bugs.
for all the sites where I had issues (and for nearly everyone else who I have read about having this kind of problem), the solution has been to update all themes and plugins.
if your theme is not abandoned, it surely have an updated version already available.
in any case you could buy an updated version of Visual Composer and install that rather than using the packaged one (you'll have to remove the packaged VC folder within the theme).