and just for the record, the software requires me to be on the same subnet to connect, right?
No, it does not. But the sub-network addressing must be valid/known. You can have a PLC on The Internet and the software can connect. The software uses basic IP Addressing, but the addressing must be known.
If something is not on the PC's subnet, it goes up 1 level to try to resolve the address (e.g. the VPN may be able to route it properly). But you have to configure the VPN to handle multiple/tiered subnets. It keeps going up until it can resolve it. The point being, somebody has to resolve the address. If the IP Address is not on any subnet, it goes to the gateway of the PC (not the PLC) to resolve the address.