You questions is unclear. Did you setup your vpn and hit some issues or do you ask in advance? Describe in detail what you expect, what you see and share logs and configs.
Generally speaking once you connect with vpn you virtually enter your network, so there should be no difference how you access internal resources.
Maybe to make it even more clear: once you are connected via vpn, you can simply use the ip-adress of the pc which is running nextcloud to connect to it.