I’m not sure why that would be. I have two subnets running on my current network, and Roon runs fine across both. Adding a vpn would simply add another subnet, as far as topology goes, it seems to me.
Perhaps I have some inter-subnet routing going on that I don’t remember setting up, but if I’m doing via Unifi on my network here, I should be able to do it via Unifi on a Unifi point-to-point vpn. We shall see.
In any event, there have been cases of vpn success, like here.