The fact that your server tells you that you exhausted your available memory of 805,306,368 byes, which does in fact equal 768MB, means that your site has an extraordinary high memory load, which I have never seen on any of my client sites.
And the second number 17,469,963,724,575,190,624 bytes, means that your site was attempting to use 15,888,839TB (that is terra bytes) in memory. Some plugin or theme on your site seems to have a very serious memory leak issue, as there is simply no conceivable WordPress setup ever needing that much memory.
Visual Composer itself is not known to have any memory leak problems, at least I never encountered one, so it is either an incompatibility issue between VC and another plugin or your theme, or something else. As a first step, you should deactivate all plugins, with the exception of VC, to see if VC might start working. If it does, reactivate the other plugins one at a time, until you find the one that might cause the conflict.
Temporarily switching your site to a standard WordPress theme (Twenty Twelve, and so forth), might also help.
Most likely, these are the steps any support staff will ask you to do anyway.