Uhm, pretty "ghetto": git init in your ~/vimwiki directory and then it's git push/pull all the way.

I guess you could automate that if you really wanted. Another idea would be to put your vimwiki folder into your Dropbox.

As vimwiki files are plain text you can really do almost anything to them.

My .vim dir and .vimrc file are symlinks into Dropbox. Dropbox versioning is good-enough for stuff like that.