Q: Why would a deacon baptize an adult? If confirmation and first communion should normally immediately follow baptism of an adult, and if an adult who has been baptized by a deacon may not be confirmed by a priest, then when would it make sense for a deacon to baptize an adult? At the vigil, if the bishop is present to confirm?
A: It would not be ideal. There are some circumstances when, due to the unavailability of a priest, a deacon baptizes an adult. For example, in parts of the world where priests are scarce or in a prison system. Normally, an adult to be baptized should be admitted to the catechumenate and receive all three sacraments of initiation together.
At the Vigil, the bishop should baptize, not a deacon.