If OS out the vbox have the capacity to assign IP address to the guest OS, as you need to connect to the Internet, so your ISP knows your natural IP address but no need the spoof one which only needed when transferring data between in and out the box. So if the IP address from OS in the box leaks, it’s the spoof one, and can set any IP address if you like (not only the Tor exit node as the current whonix does).
When you leaking, the plugins (flash, webrtc, etc) or other softwares would think themselves had positioned you, because the address is different from Tor list and the Tor exit node.
how can I have Virtualbox give the host OS a specific IP address to connect to guest OS? As the ISP doing to your host computer.