The Liturgy, while of immense importance in its own right, is a standardised service, with few variable portions in it. Vespers and Matins, however, while adhering to a basic structure, are stuffed full of the doctrine and theology of the feast they precede. The bulk of a Vigil consists of these "variable" portions. There are readings from the Old Testament, which prefigure the events of the New, there are various psalm readings, and the hymns, particularly the Canons at Matins, really get to the heart of what the Church teaches.
PM me if you want the full Vigil text in English for this Saturday night, which is the eve of Pentecost.