We have 3 lans connected via various methods with ips of 100. 172. and
10 with the 172 network acting as a hub. All three networks are
contactable from each other.

We've just added a mono firewall to the 172 net and joined it via a
vpn link to a new network, 192.
The 172 network is still able to ping and have connectivity between
all 4 sites, but the other three sites can't talk to each other as it
seems the vpn link isn't working as a bridge/forwarding packets
through the vpn link.
The default gateway on the 192 lan is to point to the mono firewall on 192.

Is this a limitation in mono, or more likely my understanding of how
this should be configured?

Andy