I am glad you are fine. Loss of breaking power (vacuum loss?) is a very serious issue and it's puzzling why BMW dealer did not identify any codes which may have bene generated.
What you are describing is very similar to the recall affecting N20 or N26 equipped vehicles. Brake vacuum pumps can fail like any other mechanical part.
http://www.autoblog.com/2013/09/28/b...-brake-recall/
Is there another BMW dealer you can visit? Charging diagnostic fee for a vehicle under warranty is also questionable.