Today I am working on a vagrant base box using gentoo and VirtualBox. I created a gentoo stage-3 vm in VirtualBox. But this is not enough to run gentoo in VirtualBox let alone as a vagrant box. There are a number of packages that need to be installed to make a stage-3 useful for virtual box and vagrant.
acpi - for poweroff
sudo - to run commands without a password
emerge --ask app-emulation/virtualbox-guest-additions
Keep in mind this package installs dbus. I recommend setting up dbus using this guide.
This is a service I did not have running on my own system. Follow this guide to get it setup. This will allow gentoo to shutdown when performing acpi shutdown in the Machine menu.