There's a canon which prohibits priests from entering the altar without the black cassock and stole and deacons from entering without alb and stole.Just a related question: does that mean Syrian clergy don't vest in the altar? I know Armenians don't, seeming to have adopted the Western sacristy to go with the Prayers at the Foot of the Altar (which necessarily come before entering the altar), but I thought the other churches did.
Before a candidate for Holy Orders is brought into the altar for the ordination, the bishop reads an exhortation which consists of an extended confession of Orthodox faith, to which the ordinand must vow obedience (along with obedience to the bishop, the Synod, etc.), and an explanation of obligations proper to the office about to be undertaken. The priest is exhorted never to enter the altar without at least the black cassock and stole, and not to let deacons enter without alb and stole. Generally, we abide by this, but another interpretation is that you cannot serve liturgically within the altar without these vestments, but may enter without them for legitimate reasons (e.g., they arrive before services begin and want to set up).
With regard to vesting for the Liturgy, however, there is a difference. The preparation of the gifts occurs in two parts: first, the bread and wine are arranged on/in the vessels and offered with prayer, and then the commemorations are read (this is the order in which it is printed in the books). But the vesting of the priest occurs as a part of the second half of the preparation, which means the bread and wine are prepared by the priest dressed only in black cassock:
After this, the commemorations are read:
But the rubrics allow for the possibility that the priest may fully vest first, before doing anything else: in this case, the priest is directed to turn to the section in the second half of the preparation with the vesting prayers, vest, and then turn back to the first half to prepare the gifts. Typically, priests will vest in front of the altar (which is covered by a veil at this point), but they can also go to the sacristy (in this context, not a Western import) and vest there. It really just depends on the priest and the available facilities.