I actually had a theory about how +Vigano converted, which he later confirmed.
When he went into hiding / seclusion after blowing the whistle on the McCarrick situation, he took advantage of the Motu to begin offering the Tridentine Mass. Nothing teaches the faith and the true sensus Catholicus than the Tridentine Mass ... which is why Bergoglio admits that he hates it so much. While +Vigano was alone, offering the Catholic Mass, and having time to reflect about it due to his seclusion, he came to consider the causes of the decay. I'm sure that his thinking was partly motivated by his repugnance for everything Bergoglio stood for. There are many in the Conciliar Church who started to wake up because of Bergoglio.