how did you configure those networks on the FGT? Is ist vlans oder secondary ips?
In any of both these cases the FGT already has the routing it needs. Still there needs to be policies to allow traffic to flow.
Also the other side of your vpn needs to know a route into A and B (reverse path that is. It needs to know how to route packets back to A and B to be able ro route replys to packets). Without that packets that go from A or B to VPN will reach their destination in VPN but you won't get anything back.
"It is a mistake to think you can solve any major problems just with potatoes." - Douglas Adams