Hi.You should use "ip helper-address" i think. I had some free minutes at my job and i've done simple gns3 emulation regarding your situation( as i understand it from your description). But it is without any vpn and so... if you have vpn between offi...

Maby you should try to add connection with your loopback thru cloud. i always connect my pc's loopback, virtual machines thru cloud.