Is Archbishop Vigano's demand that Pope Francis be judged a heretic and removed a valid argument?

The first mistake is that the maxim 'a heretic is unable to assume the papacy' is about the time before a man becomes pope, not after. Vigano is making a post hoc argument. The rule is: a man who, before being elected, had already been convicted of formal heresy by a canonical legal process, cannot validly assume the papacy. Nobody questions that. But such a man would already be excommunicated and would never be a candidate, and no conclave would elect him. You cannot take that rule about a man before he becomes pope and anachronistically apply it to a reigning pope who you think has now fallen into heresy, and declare him deposed. It does not work that way. And the word formal is decisive: to be a formal heretic you must have been judged by a canonical trial, with a prosecutor, a defender, witnesses, and full knowledge on the accused's part, the way one must have full knowledge to commit a mortal sin. There is no canonical court that can try a reigning pope, not even every cardinal in the world, because canon law is clear that the pope cannot be judged; that is reserved to Christ, and deliberately so, because if you could judge the pope as a heretic you would be placing yourself above him. The only one who can declare a pope a heretic is a later pope, after the suspected pope has died, as was done with Honorius, whom Leo II and later councils condemned after his death. So for Vigano, or for Cardinal Zen who says Francis has committed heresy, to demand this is to attack the very structure Christ built, and that is playing with fire.