You cannot get the VirtualBox GUI running in the virtual terminals 1 - 6, only 7 and 8. Basically, you need to start the X server on VT8 and make programs run on it.
If you do not need window borders and a titlebar (because you're running VirtualBox fullscreen anyway) or other fancy display manager functions like Alt + Tab, you will be fine with a simple X server. Note that this may not be compatible with user-switching or re-logging when the main X server crashed.
Start a detached X server
sudo X -quiet -nolisten tcp -noreset :4 vt8
This will switch you to VT8 (Ctrl + Alt + F8) which is initially black. Switch back to GDM/KDM?whatever at VT7 by pressing Ctrl + Alt + F7. To run Virtual Box in VT8, run:
DISPLAY=:4 virtualbox &
This command first sets the environment variable
:4 and then runs
virtualbox. You need to switch manually to VT8. Notice that you don't have a title bar nor a window border. To quit the application, use the File -> Quit option. To stop the xserver, first determine the process ID of your X server (using
ps uww -C Xorg | grep :4) and then run
sudo kill PIDHERE.
This is somehow advanced stuff, I recommend to stay using different virtual desktops (Ctrl + F<number-of-desktop> because using a second X server makes you lose sound from the main X server too. If virtualization is not a requirement, consider using