Hereās how it works at my parish where we have four deaconsā¦
A deacon is an Ordinary Minister of the Eucharist so, if a deacon is present at a Mass - regardless of whether there are other assigned deacons already vested and serving in the sanctuary - that deacon (if there is a need for extra ministers of the Eucharist besides the priest and two assigned deacons) is required to assist in the distribution of the Eucharist; and that āassistingā deacon should (at the very least) be vested in his alb, cincture and stole - 'though not necessarily in a dalmatic. (Iāve observed that this same assisting deacon should also help out as needed in purifying the ciboria, chalices and paten).
Likewise, a deacon who is present at a Mass where other deacons have already been assigned to be the Deacon of the Word and Deacon of the Eucharist, should still assist in his role as Lector, a function proper to his vocation. However, I donāt think he needs to be vested to act in his duty as a Lector/Reader (the titles, apparently, are interchangeable). That way, he can sit with his wife without drawing undue attention to himself (and sitting with oneās wife at Mass is something which is rare in the life of a deacon). My sense is that our deacons truly look forward to the occasional privilege of sitting with their wives at Mass.