Not being well-versed in Canon Law, I can only speak anecdotally. Unless vacancies on the Vestry prevents a quorum in order to conduct business, the vacancies can continue without too much concern. If, however, it does prevent a quorum, the Bishop (ususally through a "Diocesan Visitor") can select a person & appoint them to the position. Where do they find these people to appoint, but who refused to stand for election? It's surprising how many will readily agree to serve on the Vestry if the Bishop gives them a personal invitation to do so. It's the same thankless job, but suddenly they see it as an honor because "The Bish" picked them especially. Vestries Beware! This appointee is always & forever indebted to The Bish & will be obedient to his will. If it comes down to what the parish wants or what The Bish wants, this person will always vote in favor of The Bish.
Interesting point!