"Even if I were to accept the ecumenical status of Trullo, nothing in the canons that you quoted gives any evidence on the nature of the bread that was used; on this point the canons are silent."
The evidence is implicit in Canon 32. Note that the canon tells us that the Liturgy was given to St. James complete with directions, i.e. what we would call rubrics. These would surely have included not just the matter of mixing water with the wine, but other important details, e.g. what sort of bread could be used.
Note also that the Liturgy of St. James is seen as the root liturgy. All other liturgies of the Church, according to Canon 32, are derived from the Liturgy of St. James. Thus, the Liturgy of St. Mark in Alexandria is from the Liturgy of St. Peter in Rome which was from the Liturgy of St. James. How do we know? Tradition tells us that St. Mark the Evangelist helped St. Peter in his ministry to the people in Rome.
The Liturgy of St. Basil came from the Liturgy of St. Peter the Holy Apostle as celebrated in Antioch which came from that of St. James. The Liturgy of St. John Chrysostom came from St. Basil's.
All over the Church, the various liturgies used have their origin in the Liturgy of St. James. And yet, as Agrarian noted in one of his posts, nowhere in the East, except in Armenia, do we detect the use of azymes.
That's a clue. Now apply the Vincentian Canon to the evidence and the only conclusion possible, it seems to me, is that azymes were not originally employed in Rome or in Armenia.
You are assuming that whether the bread were leavened or not was an important detail. It is just as easy to see the fact that bread is bread, leavened or not. Canon 32 is silent on this and so should we be.