Vintage mode stores a value in settings that should tell you whether or not the user is in 'command mode.'
So, you could register a callback with the sublime.Settings.add_on_change() method that gets called when the setting 'command_mode' changes. Then test for the value you need and perform your operations.
sublimetext.com/docs/2/api_r ... e.Settings
I haven't tested this approach, but it's worth trying.