If you have them in bridge mode they will be transparent to your existing network. Based on that you would be able to keep your current subnets.
As I understand this you have an network with an internet connection and another network without an internet connection and they are not linked in any way.
You will not be able to just bridge the networks wirelessly unless you setup VLANs and I don't think you want to do that. You'll need to wireless radios, I'm not familiar with TP-Link devices but Ubiquiti has a few devices that would work and I presume TP-Link does too. Here's roughly what I'd do.
- Setup a wireless access point in your public facing network using radio-1.
- Setup a client radio on your private network using radio-2.
- One of your radios should be setup to route traffic and the other can be setup to bridge traffic.
- Assuming you have radio-1 setup as a router you just need to set its WAN side as the gateway on your internal lan.
- You will also need to setup a route on your public facing internet router/firewall so that it knows to talk to radio-1 to reach your internal router.
- Now you can run your VPN server in either one of your networks behind your main router/firewall.