Ok, lets say for example you have the following setup...
Public IP: 123.123.123.123
LAN IP: 192.168.1.10
When you add your LB you would use the IP of 192.168.1.10 instead of the public IP 123.123.123.123
As long as the Main Server and the LB are both on the same 192.168.1.0/24 subnet then it should work just fine.
However, your customers will not be able to connect as the Main Server would pass 192.168.1.10 as the LB IP to your customers so you then run into other issues but this can be fixed by entering your Public IP of 123.123.123.123 as the VPN IP assuming you are using XUI.