(Edit: to be clear I accept Pope Francis to be the pope, and do not adhere to Vigano’s statements to the contrary. But I do think it is an open, debatable question. I am no canon lawyer or theologian, but I expect the experts in support of Vigano to be considering the following arguments)
1. Lack of due process - notifying him merely by email of the accusation, no trial giving him a chance to clarify or defend his statements. In particular, since his statements can be interpreted in a variety of ways, including in keeping with the Faith and unity with the Church.
2. Political motivation/double standard - not excommunicating the German bishops formally teaching heresy and approving heretical disciplines, while only excommunicating a single bishop calling out the very same heretical political agenda behind the election of Bergoglio. That is the agenda of The St. Gallen Mafia to approve communion-for-the-divorced-and-remarried, blessing of homosexual couples, female deacons, etc, etc, etc.
3. Vigano’s statements did not rise to the level of a) formally, officially judging the validity of Pope Francis, as he already stated, b) formal, legal break with the pope, c) interpreting the Council anymore strictly than Archbishop Lefebvre or the SSPX.
4. Lack of motive - clearly he had no motive to deny in principle submission to a validly elected pope, or in practice in a formal, definite way. Rather, as he stated many times, his motive for his declarations was to defend the Faith and the Church from the evils coming out of Rome since the election of Bergoglio. Even if there were no such evils (there clearly are), he acted in good faith believing there were.
5. Case of necessity - of the 5000 Catholic bishops, Archbishop Vigano was one of a tiny number of bishops speaking out publicly on the corruptions and evils coming from under this pontificate, from Rome, which he summarized in his last statement before his sentence. The whole Church needed bishops to speak out publicly about the evil agenda, in order to defend the Church. Even if Vigano spoke unwisely at times, which is debatable, clearly he spoke out out of necessity (real or perceived), out of love for the Church, as a Successor to the Apostles.
6. Disproportionate punishment - excommunication is the worst possible punishment. Considering Vigano’s age (84), decades of service to the Vatican and as US papal nuncio, even if he was guilty of singular acts of schismatic statements, those are singular acts. He could first receive a) a formal public warning, b) censure, c) suspension, d) fine, e) being forbidden to speak publicly, etc.
7. The judgment of Cardinal Fernandez, overseeing the DDF presumably issuing this judgment, cannot be trusted due to a) issuing permission to bless homosexual couples (as such, which he did clarify), b) not repenting of his pornographic and heretical book written as a priest including a chapter with child pornography involving a child having sexual relations with Christ c) being the chief author of Amoris L. giving approval for active adulterers to receive communion, active homosexuals to have liturgical roles, etc.
8. The evidence as presented that Cardinal Bergoglio was not validly elected pope (see book Dictator Pope, etc). Even if that evidence is deemed inconclusive, or objectively weak, obviously Vigano acted in good faith believing the evidence was conclusive and strong enough to make the statements he made, which were at most his personal opinion, not an official, authoritative judgment as he has already clarified.