Confused about Vim, wiki's, or Google? You are welcome. Although vimwiki is a wiki implementation within Vim, it passes off a lot of cool things that bleed into markdown based writing. However, it often only works on .md files. This is a pain point since some things use .markdown files (gollum) or .mdown files. Vim will filetype highlight things using both of these filetypes however it won't sugar it up with Vimwiki syntax.
Wrong...well, sorta. Vimwiki drags around a couple of config flags that can be used to syntax-up Vim when using all types of common markdown extensions. And....:
let g:vimwiki_ext2syntax = {'.md',: 'markdown', '.markdown': 'markdown', '.mdown': 'markdown'}
gist
Boom! Vimwiki syntax in all your markdown files. Be happy, write in markdown.