SilenceZone is the perfect app to help you managing when to put your phone in vibration or sound mode.
With SilenceZone all you have to do is to define the areas where you need to have your phone in vibration mode and the app will warn you when you get there. It also reminds you to put the phone back in sound mode once you exit the region.
This way youll never ever have to handle those embarassing situations when your phone rings in the middle of a meeting or in a class.