Your wish is granted: https://github.com/ryanss/vim-hackernews
179 karma · joined July 1, 2013
It's just a single, well-commented .vimrc file a little over 100 lines long. The main plugins that add to the visual appearance are the jellybean colorscheme and the fancy status line comes from vim-airline.
It's not currently possible to post comments using the official API (or any of the unofficial APIs), but this probably wouldn't be difficult to implement using a standard POST request to the actual Hacker News web server. The harder part would be getting the interface for posting comments inside of Vim user friendly; it would probably be similar to how fugitive.vim lets you enter git commit messages.
I will definitely look into adding this to the plugin if enough interest is shown.