hello
to answer your question.
you could download the gem file and then try
vagrant plugin install <vagrant-vbguest.gem>
however the only thing this plugin does is to reinstall virtualbox guest additions if not detected, the operation on the vm may need internet, as the OS may need packages (perl gcc kernel-headers kernel-sources)
so i think will be better if you create your own box ready to use, and then on the machines without internet access you just use them
this will also make happier the end user as will be quicker
you can use packer tool to build boxes, and you can use bento boxes as sample starting points
alvaro