Well, I might be a sorta happy camper. Looks like iVim is a decent port of Vi IMproved to iOS. From what I can tell, seems like a rather old (7.4) version, compiled as Big with external scripting and various mice/gui things disabled.
Limitations seem to be principally iOS imposed ones, such as Extreme Sudden App Kill Syndrome and overly restricted file permissions. So in effect, it’s about as good as you can hope for on anything more fruity than a Mac.
On a related note, I can also say that iPadOS doesn’t do key repeat. E.g. holding j doesn’t move the cursor in iVim, nor does it insert a bunch of j’s into Safari. But the repeat stuff works fine when combined with a modifier like doing