Is it customary for Deacon and Subdeacon to say Vesting Prayers?
Why not? the prayers for the Dalmatic and Tunicle are already provided among the prayers to be said by a bishop when vesting, and I suspect they are of some antiquity because they express the first-millennium idea that these vestments are signs of joy (which is why during penitential seasons they are replaced by folded chasubles - find my treatment of that via the Archive facility). Oh yes ... and can I ask ... am I the only person who puts the Maniple on after the Dalmatic for fear that otherwise my left arm will get helplessly entangled? Come to think of it, the Maniple Prayer doesn't go particularly well with the Dalmatic Prayer.
While we're on Vesting Prayers ... I've always envied pontiffs the prayer said while taking off the Cappa: Undress me, O Lord, of the Old Man with his morals and activities ... There ought to be another prayer (I hope someone would like to compose one, preferably in Latin) for the pontiff to say, after Mass, as he again puts on the Old Adam and goes back to his ordinary everyday life of murdering, fornicating, and embezzling.