A short tip: Markdown doesn’t have any form of official or non-printable markup in its language definition (that I could ascertain) so adding a Vim modeline into a document needs a little bit of massaging. You could include the modeline inside a HTML comment, but the better option that I found was to do this at the end of the document:

[modeline]: # ( vim: set fenc=utf-8 spell spl=en: )

This uses the link label syntax to establish a link called modeline, point that at the URL of #, and then use the title of that fake link to set our Vim modeline, following the modeline syntax and appropriate spacing and colons.

That’s it. Hopefully this saves you some thinking.