Vimium dev here. We actually disabled Vimium on Google Reader because we were getting complaints about how it breaks Reader's own keybindings. If you wish, you can re-enable Vimium by going to the options page; once enabled, its keybindings should take priority over the page's own -- please file a bug if that isn't the case. (However, if you pressed a key that doesn't correspond to a Vimium command, it will automatically pass the keystroke down to the page's own key handlers.)