Vigils were traditionally days of fasting in preparation for the feasting to follow, an approach worthy of consideration!
The dogma of the Assumption - that Our Lady was assumed body and soul into heaven - was formally defined nearly fifty years ago, on 1 November 1950, in the Apostolic Constitution Munificentissimus Deus.
The feast itself of course (and its Eastern equivalent, the Dormition of Mary), which attests to the longstanding belief in the dogma, is of very ancient origin indeed. One of the ancient traditions holds that all of the Apostles were recalled from their various missions to be present for Mary's 'falling asleep' (ie death) at Jerusalem, around the year 49 (around the time of the Council of Jerusalem, chronicled in Acts). Certainly of all the feasts of Our Lady, it is this one that has garnered the most elaborate traditions, Offices and musical works.