The existence of error in Vatican II does not render the promises of Christ void. The Church does not genuinely teach Vatican II errors since the Church is perfect and therefore never contradict herself. Besides, Pope John XXIII declared in his opening address at Vatican II that the council would define no new doctrine. Therefore, the subsequent attempts to define new doctrine could not have enjoyed the protection of the Holy Spirit.
I found Fr. Most’s reconciliation of Vatican II’s teaching with the pre-Vatican II magisterium quite persuasive.
But then how certain bishops have twisted it becomes quite another issue.
Vatican II was an act of the ordinary magisterium to borrow the Scholastic term. It provided an authoritative interpretation of existing Catholic dogma.