Forward VPN traffic to another server

First you need to establish a VPN link between server A and B, let's say server B will get 10.10.0.1 as address, server A 10.10.0.2 and the virtual interface on server A will be called tun1. You can use whichever technology you fill more comfortable.

Then, as remarked by davidgo, you need to use policy based routing on server A. In order to do so:

1. Add a new routing table to the file /etc/iproute2/rt_tables:

   200 vpn
   

2. Add a selection rule to use the routing table vpn for all traffic coming from the private network:

   ip rule add from 10.0.0.0/8 table vpn
   

3. Fill the routing table

   ip route add 10.8.0.0/16 dev tun0 src 10.8.0.1 table vpn
   ip route add 10.9.0.0/16 dev wg0 src 10.9.0.1 table vpn
   ip route add 10.10.0.0/16 dev tun1 src 10.10.0.2 table vpn
   ip route add default via 10.10.0.1 dev tun1 table vpn