I use vimium myself, so I was interested in this too.
Look for "Extensions on chrome:// URLs", enable it. Then click on 'Relaunch Chrome' to apply the change.
This only allows an extension to be used in chrome:// URLs IF the extension requests this permission.
But it's a step in the right direction. I'm not sure if Vimium or Vrome (two nice vim keystroke extensions) ask for this permission. Haven't tried this yet.
And if you changed anything ELSE while you were poking around in the flags... Well, be careful!
EDIT: Tried it with Vrome, still doesn't work. Evidently no one is asking for this permission.
And Chrome complains about the Extensions in Chrome:// URL's being enabled. Bummer.