You will need some device to act as a router between all the VLANs, this could be a high end network switch as an example. Whatever is used needs configuring to know the static 'route' via which to reach the VPN subnet.
As an example I have in the past also used Apple's VPN server, the VPN clients where in a dedicated VLAN and therefore their own subnet. I also had a hardware network firewall which was the main default gateway for the LAN. All traffic to the Internet obviously had to go via the Firewall. For normal LAN devices this was straight forward as they used the FireWall as the default gateway and this also applied to the LAN ports of servers also on the LAN. However as the VPN clients were on a separate subnet the Firewall needed to know that in order to reach them i.e. to route to them it needed to send all traffic to the LAN port of the VPN server, this therefore require defining a static route in the Firewall.
Without knowing how all your VLANs are linked I cannot definitively say where you need to define the static route or routes but as mentioned if they are all being created in a network switch then the network switch might be the logical place to do this. Again as an example I had a network with a VLAN for computer and a VLAN for VoIP phones and the switch acted as the router between the VLANs.
It would not typically be on the Mac server itself you need to define the route but rather whatever is the default gateway for the network or networks.
