Seems you are using an old emacs/vc/vc-hg version. Simply upgrading is probably the easiest solution.
I had a similar problem, which I could solve bit by bit by improving small settings and even made a patch. It never really solved all. Until I upgrade to a recent emacs (and support file); I use the Aquamacs distribution now, which works great on the Mac. (Although it’s keybindings are more mac the traditional-emacs)