A single openvpn server allows multiple VPN clients. Assign each VPN client with a fixed IP address (ie the address on vtun) , so each remote site has dedicated vtun address.
Routing protocol like OSPF might be usefull to learn routes to each customers network, without having to assign them manually on hub router.
On ER-X , masquerade might be usefull. This way, hosts on remote networks do not need a route back to your central site