You don't need the VPN license because the VPN doe'sn't start in the IP-Office but in the external box,
There a actually two app notes:
US tech tip 015 VSU configuration and IP-Office
IP Office 403 and SG VPN Application in the /mergedProjects/manuals/other/application_notes.htm section of the documentation CD.
I used the first one to have a remote lan for PC's and a 4620, works fine