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.
Excellent point. I had forgotten about this. I work for a non profit so I'm ok, but yes you should absolutely check the terms of the license before using. On the upside, almost everything is markdown files in regular folders, so you can fall back to vim anytime.