hum, odd, just to make sure, you are running 18.04 as guest in a vbox that is on a 16.04 host, right?
and when you run guvcview in your 18.04 guest, you see video from JeVois no problem?
can you try to run jevois-inventor under sudo to see whether there still is some permission issue? Also, did you configure your vbox USB devices such that all three of the video camera device, serial port, and mass storage device exported by JeVois are passed through to the guest OS?