Thank you for taking the time to reply. The initial setup was a little tricky to get my head round, but with your encouragement, I've done some more digging and can now see what's going on.
You are, of course, correct, and I was being distracted, by, among other things, the wizard.
This looks to be a great addon. Thank you all the hard work that went into producing it.
Just to follow up. I have it all working, including the DNS up and down scripts referenced here: https://github.com/Zomboided/service.vpn.manager/issues/282 but using the --up and --down options just in the opvn files that need them rather than in the addon settings.
Works a treat!