This is really neat.

And AFAIK not something that Vim does well. At least, in my vim setup, nested modes are clumsy.

Code-inside-markdown means Markdown has to support the syntax (and snippets and shortcuts etc). Or requires a entire language (or dialect) like erb being both Ruby and HTML which cannot leverage the ruby and HTML at the same time, AFAIK.

What is, or are the common vim solution for nested or mixed modes?

This is a good read written by someone who uses multi mode editing for productive reasons:

http://howardism.org/Technical/Emacs/literate-devops.html

Oh yeah, I found his http://www.howardism.org/Technical/Emacs/spreadsheet.html about org-mode and spreadsheets, yet another thing that I love about emacs and can't imagine a "modern" IDE doing as well.