I'm sure that somebody will answer this better than me, but the short answer is you need a router, a device which connects two different networks. 192.168.1 and 10.1.1 are two different networks.
However, because you are trying to connect these over a VPN, that suggests there is something between them, which might be a router, or might be something which can become a router (namely a server) - can you explain how the VPN works - i.e. where are the two networks physically and how are they connected?
The "easy" way around this, I would think would be to change the IPs on the PLCs, but you obviously have a reason for not wanting to do that. As for spending money, if a router is the answer they are available very cheap - if hardware is the answer, someone on here will advise better than me which one you should get.
Sorry that isn't too helpful, but try to explain how the two networks are set up and maybe someone can advise better on how you can join them together.
What I can tell you is there is no way you can have 10.1.1 computers and 192.168 computers on the same LAN able to talk to each other. Unless someone here can correct me?