If you have Vim optimized for coding, probably you’ve problem for writing on it. The first one that comes out for me as soon as I started to writing markdown on Vim was the word wrap question, any sentence comes longer easily and you can’t read it anymore if you have set Vim to not wrap your code.

If you want to configure your Vim to wrap long lines only in markdown files, all you have to do is to configure this on your dotfile:

au BufRead,BufNewFile *.md setlocal wrap

What this magic command do is: when either opening or creating a markdown file, set the wrap command only for this buffer.

UPDATE: Apparently, there are better ways to aim that :

augroup Markdown
  autocmd!
  autocmd FileType markdown set wrap
augroup END

From the :help autocmd:

:autocmd adds to the list of autocommands regardless of whether they are already present. When your .vimrc file is sourced twice, the autocommands will appear twice. To avoid this, define your autocommands in a group.

Or, you can just create a filetype plugin with:

" after/ftplugin/markdown.vim
set wrap