Thanks other8026, good pointer. There was already one link Pokemon Go was set up to handle, and three more as options. That gave me hope, but unfortunately it didn't fix the problem.
It has started me on another track though - if I could see exactly what Vanadium is trying to do, there might be another link I find I need to set to open with Pokemon Go.
It looks like I might need to examine the system logs for that, and it looks like I'd need to use adb.
I'll look in to that and see how I go, thanks again!
(If anyone has any easier ways to see what URLs Vanadium is navigating to, please let me know.)