I've used Vim to write most of the prose I've ever written, and I use my default programmer-oriented configuration for writing. There's now a growing collection of plugins designed to help prose writers who favour Vim, so I've got an alternate "writing mode" vimrc.
Reed Esau has created some great writing plugins for Vim. One is vim-pencil. It adds better support for text formats like Markdown, soft line wrap versus hard line breaks, and wrap mode autodetection based on the modeline. Another cool feature is Pencil creates undo points when certain punctuation characters are used in Insert mode. This is important because writing prose doesn't lend itself to switching out of Insert mode as much as programming.
Other writer-focused plugins by Reed include vim-lexical, which improves thesaurus and dictionary completion, vim-textobj-quote for coping with typographic quote characters, and vim-textobj-sentence, which makes the native sentence detection better.
vim-wordy is probably my favourite. It uses Vim's built-in spell check to detect commonly misused words and phrases. It'll help you to avoid weak or ambiguous words, but also recognises passive voice.
To invoke Wordy, type
:Wordy with a dictionary name. Press tab to see the list of dictionaries. Good ones to try are
:Wordy problematic and
:Wordy passive-voice. Wordy is not a grammar checker, but it helps avoid common mistakes.