- Instead of .vimrc, the filename is _vimrc - because Windows doesn't like filenames beginning with a dot.
- Instead of .vim, the directory name it is looking for may be vimfiles - for the same reason.
- Depending on the configuration of Windows, it may not be looking for your files where you think it is - to find out you can do :echo &rtp
Now I know where to put this useful snippet that makes the markdown editor work with Jekyll's YAML frontmatter. Phew.