You generally don't have to open/forward ports on your own router...to VPN from that routers protected network...out across the internet to another network.
First...does you offices network have a functional VPN..that's been tested before..and is working from other than your home?
Generally you want to make sure your router has the most recent firmware installed
In most routers...there is a section to which deals with "VPN Passthrough"...both PPTP, and IPSec. (those are the 2x common types of VPN). Some brands have these enabled by default, others don't. I can't remember what Belkin does..but you make want to poke around your routers web admin...and look for those.
PPTP uses Windows native VPN dialer..IPSec is more corporate...usually you use a 3rd party VPN dialer like Cisco or Sonicwall, etc
Lastly, your home network cannot be the same IP range as your office. Meaning, if your office is 192.168.1.xxx..and your home network is also 192.168.1.xxx...no go. Must change one of them...make it something like 192.168.0.xxx or 192.168.2.xxx, etc. Easily done..log into the router..change the LAN IP...save settings..reboot your machine(s) to pickup the new IP.