A festal vigil is a series of services held on the eve of a feast. In churches following Russian usage, this usually consists of Great Vespers, Matins, and the First Hour. For the Nativity, Theophany, and the Annunciation, if these feasts do not fall on a Sunday, then the Vigil is made up of Great Compline, Vespers from the Litia onwards (including blessing of bread, wine and oil), Matins and First Hour. In my experience, these vigils run for about two and a half hours, but this would, of course, vary from parish to parish. Ask your priest for a more accurate answer.
will he need me to serve in the altar?
From what I know, there's less for an altar server to do during a vigil than there is during a Divine Liturgy, but altar servers are still needed, particularly at the blessing of the bread, and at the Polyeleos and Magnification. Again, ask your priest for more details.