While the browser extension can be locked with a keyboard combination, the app does not appear to have such a feature.
On Android there is already a “Panic” app by the GuardianProject, which offers a trigger for locking, logging, out or even hiding certain apps (if they implement a receiver).
I think that this would be a great addition as it would allow users to quickly “secure” their vault before handing their phone to someone else.
Do you think the PIN feature on the app is NOT secure enough? You can set a fairly long PIN and then the app will only give 5 attempts before logging out the person entering > 5 PIN codes!!