the differences between the Flemish school and the Second Viennese School are enormous. The expression Second Viennese School generally refers to Schoenberg, Berg, and Webern. In reality, it could also refer to other composers and theorists; Erwin Ratz comes to mind, at least, who took lessons from Schoenberg and edited some of his works. The Vienna school existed because there was a teacher and there were students, there was a strong shared technical background, which sometimes resulted in a shared style and sometimes not, but which led to a substantial "homogeneity" of language. And they were all active during the same period.
The Flemish school, called Franco-Flemish by some and "the Netherlanders" by others, has never been a school. There are no educational centers of any kind, no true leading composers, no recognizable style. There are geographical connections, some teacher-student relationships, and, from what I know, much of the repertoire was all passed down through the same manuscripts. Hence our habit of speaking of this group of composers as a school.
Dufay was from Cambrai, now in France but at the time in Burgundy, but he traveled extensively, especially in his youth. We do not know where Binchois was from and it does not seem he was ever a pupil of Dufay. And he writes mostly secular works, while Dufay writes mostly sacred works. Certainly, the Burgundian court was full of literate professionals and musicians, but I would not define it as a school so much as a court. Simply.
Ockeghem was not born in Burgundy, does not work for the Burgundian court, and we do not know whose pupil he was. His style is very different from that of all the composers before him and many composers after him. And even though half the world wrote him laments for the dead, we really do not know if he was a teacher to anyone, where or how.
From the arrival of Josquin, things become absolutely simpler. Regardless of place of birth, practically ALL composers traditionally called Flemish work in Italy. They have, as usual, all different styles, write different forms, and are active in an incredible number of places (Ferrara, Mantua, Bologna, Naples, Milan, Florence, the Burgundian court, the French court etc. etc...).
I understand the didactic simplification, but only to a certain point. And if Josquin and Lasso are not placed alongside them...