First of all, AppImageAssistant has been replaced by appimagetool. I should probably update the presentation soon.
Then, keep in mind that VirtualBox is not a normal userland application in the traditional sense since it needs root rights and a kernel module. This kernel module is compiled(!) during installation by the installer, or provided by your distribution.
To make an AppImage of VirtualBox, one would need to write a script that does the kernel module compilation, and it would also require root rights. So far I haven’t attempted doing it; way too much hassle for my taste. But I am definitely looking for someone to take on the challenge!