I don’t know if this will work for you, and I’m not sure if you’re only looking for TUI editors, but Obsidian has vi key bindings and a lot of plugins.
Disclaimer: I have not tried the vi key bindings in Obsidian.
Another one I use is vscode. It has a ton of markdown plugins and vi key bindings. It also has a nice preview window.
You don’t say what type of programming you do, or if you’re just learning or developing professionally, but maybe something fully online would work? For example repl.it, stackblitz, or Visual Studio Code for the Web.
I second the suggestion for getting a Bluetooth keyboard for the iPad.