You can press shift + esc to disable Vimperator on pages and insert to enable it again.
This is not good solution as often times you wan to disable Vimperator on a few pages
but still have it enabled on other pages.
You can achieve this by configurating the ~/.vimperatorrc file.
Below is an example of disabling Vimperator on Google main, calendar, reader and tasks.
autocmd LocationChange .* js modes.passAllKeys = false
autocmd LocationChange mail\\.google\\.com js modes.passAllKeys = true
autocmd LocationChange www\\.google\\.com/calendar js modes.passAllKeys = true
autocmd LocationChange www\\.google\\.com/reader js modes.passAllKeys = true
autocmd LocationChange mail\\.google\\.com/tasks js modes.passAllKeys = false
Instead of using autocmd,
you can also use ignorekeys.
Below is an exampel of disabling Vimperator on Yahoo and Google mail.
ignorekeys add mail.yahoo.com
ignorekeys add mail.google.com