Thanks AMDGJMJ
Where did you get this from?
Archbishop Lefebvre warned the faithful to stay away from Thuc that he has caused scandal.

Once again my apologies.  It was not Archbishop Lefevbre directly who supported Archbishop Thuc.  It was Don Antonio de Castro-Mayer.  He was one of the co-consecrator bishops at the SSPX consecration of Bishop Williamson and the other 3 bishops.  So, part of the SSPX connections but not the SSPX directly...

"In 1985 one of my confr�res, the Rev. Donald J. Sanborn, suggested that our group approach Don Antonio de Castro-Mayer, the retired Bishop of Campos, Brazil, to see if he’d be willing to ordain priests for us, or at least offer some advice. This prelate had taken a strong stand against the New Mass, and his position on John Paul II was said to be much harder than Abp. Lefebvre’s.

      Father Sanborn visited Campos in April 1985 and spoke at great length with Bp. Mayer. The bishop, it turned out, confined his apostolate to Brazil.

      When Father Sanborn broached the topic of who could ordain priests for us, Bp. Mayer said: “Go to Gu�rard!”

      Father Sanborn said that he doubted the validity of Bp. Gu�rard’s episcopal consecration. The bishop replied: “If it’s valid for Gu�rard, it’s valid for me.” Father Sanborn explained some of his hesitations. Bp. Mayer answered: “Gu�rard is the most qualified person in the world to determine if the consecration was valid.”"

https://traditionalmass.org/articles/article.php?id=60...

This is the first I'm hearing of any excommunication by Paul VI. I'm not familiar with the website included in your original post.  Is the info accurate?

I have heard of the excommunication in many places.  It makes sense that he would be excommunicated sort of like Lefebvre, right?

Here is a newspaper clipping of the 2nd excommunication of Archbishop Thuc. The first excommunication was because of the palmarians:

https://www.nytimes.com/1983/04/08/world/around-the-world-vietnamese-archbishop-excommunicated-again.html