Hi… If you just want to make a very simple, manual backup copy periodically, then I believe cross-region volume forking will work.
“Fork” in this context is a one-time copy, not an ongoing connection, so the operation will have to be repeated (manually) every so often. And older spares will likewise need to be removed.
The NRT copy will not have a Machine attached; it will just be the data. That way, if something later happens to your HKG server, you will be able to restore from the rescue volume.
Alternatively, you can get a little more elegant with this by creating a separate application that just does rsync every hour, on a timer.
Both of these approaches assume that you’re ok with your Vaultwarden interface being down for a few hours (until you manually restore it) on rare occasions, 
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Hope this helps a little!
Yes, Volumes are by far and away the most confusing aspect of the Fly.io platform. (Even users who have been here way longer than I have are taken by surprise sometimes.)
They’re really only “half” of what people have come to expect from a cloud environment; I expect that the passage you quoted will eventually be reworded to instead say “use Tigris, Supabase, LiteFS, or Tarsnap—unless you have a POSIX wizard hat”.
We’ll see…