But here some one perhaps will ask, Since the canon of Scripture is complete, and sufficient of itself for everything, and more than sufficient, what need is there to join with it the authority of theChurch's interpretation? For this reason—because, owing to the depth of Holy Scripture, all do not accept it in one and the same sense, but one understands its words in one way, another in another; so that it seems to be capable of as many interpretations as there are interpreters. For Novatian expounds it one way, Sabellius another, Donatus another, Arius, Eunomius, Macedonius, another, Photinus, Apollinaris, Priscillian, another, Iovinian, Pelagius, Celestius, another, lastly, Nestorius another. Therefore, it is very necessary, on account of so great intricacies of such various error, that the rule for the right understanding of the prophets and apostles should be framed in accordance with the standard of Ecclesiastical and Catholic interpretation.
Moreover, in the Catholic Church itself, all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. For that is truly and in the strictest sense
Catholic, which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient [unanimous] definitions and determinations of all, or at the least of almost all priests and doctors.
Besides being a good apologetic for the Faith contra sola scriptura, St. Vincent gives us three criteria for making sure that even within the Church we are adhering to the Catholic faith "in the strictest sense," which he says is done by following three principles: universality, antiquity and consent.
Universality - Is what we believe what is believed by all Catholics everywhere?
Antiquity- Is what is believed what the Fathers and our "holy ancestors" understood and believed?
Consent- Is what is believed from antiquity that which the all, or "at least almost all" of the Fathers or held, or is it a minority opinion or an isolated case?
In chapter 5 of the same work, St. Vincent tells us that, because of the universality and antiquity of the Faith, when one stands up for the faith professed by the martyrs and apostles (that is, the ancient Church), one is not merely defending one part of the Church but the Church entire, as St. Vincent explains:
But in this divine virtue, as we may call it, exhibited by these Confessors, we must note especially that the defence which they then undertook in appealing to the Ancient Church, was the defence, not of a part, but of the whole body. For it was not right that men of such eminence should uphold with so huge an effort the vague and conflicting notions of one or two men, or should exert themselves in the defence of some ill-advised combination of some petty province; but adhering to the decrees and definitions of the universal priesthood of Holy Church, the heirs of Apostolic and Catholic truth, they chose rather to deliver up themselves than to betray the faith of universality and antiquity. For which cause they were deemed worthy of so great glory as not only to be accounted Confessors, but rightly, and deservedly to be accounted foremost among Confessors.
You can read St. Vincent's Commonitorium at New Advent here.