Two servers cannot have the same IP address. When you are using VPN you are going over the Internet which means the IP address you try to connect to will have to be a valid, (not 10.x.x.x or 192.168.x ...
IP reservation is the process of assigning a specific IP address to a device, such as a computer, so it receives the same address each time it (re)connects to the network, such as when you restart it ...