Good question. It has been raised partially on other threads. Vatican II was surely a pastoral disaster, as seen by the collapse of vocations among other things, but in my opinion, not heresy, no. Unless someone wants to explain how practically the entire episcopate could defect into heresy, strictly so called, in 1965. Cardinal Franzelin explains that although the episcopate lacks the active component of infallibility by itself, it is still indefectible in the sense that it can never completely defect or fail, "neither the whole body of the Church in its belief, nor the whole Episcopate in its teaching, can depart from the faith handed down and fall into heresy, because this permanence of the Spirit of truth in the Church, the kingdom and spouse and body of Christ, is included in the very promise and institution of the indefectibility of the Church *for all days* even to the consummation of the world. The same is to be said, by the same reasoning, for the unity of communion against a universal schism, as for the truth of the faith against heresy. "
Likewise Wilhelm and Scannell, "The Indefectibility of the Teaching Body is at the same time a condition and a consequence of the Indefectibility of the Church ... the Teaching Body as a whole could not die or fail without irreparably destroying the continuity of authentic testimony. " This indefectibility of the ecclesia docens, of the entire teaching body, is the same principle Pope Pius IX appeals to in refuting the claims of the Old Catholics, "they boldly affirm that the Roman Pontiff and all the bishops, the priests and the people conjoined with him in the unity of faith and communion fell into heresy when they approved and professed the definitions of the Ecuмenical Vatican Council. Therefore they deny also the indefectibility of the Church and blasphemously declare that it has perished throughout the world and that its visible Head and the bishops have erred.", in short, indefectibility precludes it. An AER article in 1965 says the same.
If the original schemas developed by the preparatory commission and approved by +Lefebvre, +Ottaviani and other prelates had passed, the Church would have reaped great fruits from this Council, Her missionary work would have continued, Her vocations would have flourished, the faith of Her children would have been nourished, errors would have been condemned and reproved. Instead, the liberals threw them out, introduced new texts of their own and the few orthodox and traditional prelates had to work overtime just to correct or limit outright heresies and grave errors, without complete success. And the disastrous results were immediately evident and remain so today. Most texts are weak, ambiguous and the possibility of lesser error cannot be ruled out a priori, since extraordinary Magisterial authority which normally safeguards Ecuмenical Councils was not invoked.
As Archbishop Lefebvre said to Cardinal Ottaviani in 1966, On all these fundamental points the traditional doctrine was clear and unanimously taught in Catholic universities. Now, numerous texts of the Council on these truths will henceforward permit doubt to be cast upon them. The consequences of this have rapidly been drawn and applied in the life of the Church:
> doubts about the necessity of the Church and the sacraments lead to the disappearance of priestly vocations,
> doubts on the necessity for and nature of the "conversion" of every soul involve the disappearance of religious vocations, the destruction of traditional spirituality in the novitiates, and the uselessness of the missions,
> doubts on the lawfulness of authority and the need for obedience, caused by the exaltation of human dignity, the autonomy of conscience and liberty, are unsettling all societies beginning with the Church - religious societies, dioceses, secular society, the family.