The Christmas Vigil Mass will be exactly like any other Sunday Mass. The priest may want to use incense. If this is the case then one server will need to act as a “thurifer” meaning he handles the thurible (the censor hung on a chain where the incense is burned). In a perfect situation, this would be his only job and he would process in and out with the thurible during the entrance and exit processions.
When incense is used at a Mass it is typically used at the beginning of the Mass before the Collect (opening prayer), at the Gospel (used by the Deacon, Priest, or Bishop to incense the Book of Gospels), and at the Offertory to incense the Gifts, the Altar, and usually the Crucifix and Nativity Scene). It is sometimes used by the thurifer or deacon to incenses the Blessed Sacrament during the Elevation of the Host and Chalice during the Eucharistic Prayer.
Besides the role of the thurifer, there are no other changes to the roles of the other servers.
On a side note (sorry for being picky). The term “acolyte” has two meanings within the Church. The first, which is now rather extinct, refers to the servers who attend to candles during the liturgy (in the processions and at the Gospel) in the Tridentine Mass. The other term, which is more widely used today refers to a person who has been “duly instituted” as an acolyte. Many years ago the acolyte (and lector) were Minor Orders within the Church and were actually “ordained.” Now however, these men are simply instituted as part of their journey to the priesthood or diaconate. Duly instituted acotlyes are the only other people besides clergy who can licitly purify the Sacred Vessels after they have touched the Blessed Sacrament.
Have a very Blessed and Merry Christmas!