I wrote a manuscript in vim a couple Novembers ago, for NaNoWriMo. I used a couple plugins, primarily Goyo [1] to add some margins, but otherwise, yeah, plain vim.
I don't think it was really any more productive than my current workflow in Obsidian. Vim keybindings are more useful for editing than for writing (and for editing code in particular, where the changes you're making are much more structured). The extra features afforded by Obsidian don't really make a difference during the actual writing process, but I find they're really useful for outlines and other preliminary work, which is something of a point against a vim-only workflow unless you want to use vimwiki [2] or something.
Granted, Obsidian is still a markdown-based tool, so there's still some level of minimalism going on there, but by that point we're really discussing markup vs word processors, which is its own conversation—and to my mind, a much more important one. I much prefer working in markdown (via pandoc) than in a word processor, because plain text is easy to edit and process through the terminal, and because it lets me separate style choices from content.
I find that the markdown live preview that editors like Obsidian and Typora provide (and which vim doesn't) is a really nice compromise between a slick composing experience and the technical affordances of markup. Between that and Obsidian's hypertext features, I think I'll stick with Obsidian for the foreseeable future.
[1]: https://github.com/junegunn/goyo.vim
[2]: https://vimwiki.github.io/