This is a strange request, but I understand what you’re thinking.
Your first problem is that this goes somewhat the opposite way of how VPN’s usually work: You connect your client to a VPN to hide your IP while connecting to servers. You want to do the opposite, hide the real IP of the server from the clients. The IPaddress of the server is somewhat crucial for a client to be able to connect.
Fortunately there exist a system that do what you want, but it is usually not used as such in VPN’s: NAT. This is the system your internet gateway use to allow your entire internal network on to the internet using only one IP address. This is also where we need a system with port forwarding to allow an external IP access a LAN address. We need to tell the gateway where to internally send a request arriving from an external address
If you can find a VPN that has the possibility to set up port forwarding on the one IP adress they provide you, so that the VPN address port forwards a request to your middle VPN address, so that you again can port forward that request to your servers LAN address we’re there.
If you have a good gateway you can probaly ask that device to connect to the VPN, so that your MineOS server only needs to handle LAN traffic. If your gateway is unable to connect to external VPN’s you need to get your MineOS server to to the connection. But this is entirely dependent on your chosen VPN’s ability to port forward requests from adresses outside the VPN (your clients) to your internal VPN address. As I said, this is normally not how this is done.
Other ways of hiding internal ip-adresses is bu use of proxies and loadsharing servers, that passes on requests to servers. Since this is systems not really meant to hide adresses, your IP might still be revealed by using something like trace route applications. And you would need to find an external server willing to pass requests to your server.