Both books straddle the line between fact and fiction, something of a trend in the literary world these days, and are the fruit of meticulous research by their authors.
Vuillard's 160-page L'Ordre du Jour (The Agenda) focuses on Hitler's 1938 annexation of Austria, the Anschluss, but also looks back at a meeting of Germany's top industrialists with the soon-to-be chancellor and his lieutenant Hermann Goering in 1933.
There the heads of 24 companies that are still household names today - Opel, Siemens, Krupp, BASF, Bayer - agreed to finance the Nazi Party as it headed for power.
Vuillard also picks out little-mentioned details of the Anscluss - German tanks, the very symbol of the might of the German war machine, broke down, causing traffic jams on the road to Vienna, and young Austrian women turned out to give the Nazi troops an enthusiastic welcome.
The 160-page book was considered an outsider on the Goncourt list, partly because some critics have challenged its right to be described as fiction, partly because Culture Minister Françoise Nyssen was occupied a leading position at its publisher, Actes Sud, before joining the government.
It won against three other novels after three votes.
But Goncourt jury chairman, Bernard Pivot - one of France's best-known cultural figures - hailed a "lesson in political morality" that shows that "a small group of people, with bluff, intimidation and brutality can manage to get round a country and set off worldwide catastrophe".
Vuillard, who is also a screenwriter and director, specialises in close-up looks at history in his fiction.
Subjects of his previous books include the 1885 conference where the European powers divided up Africa between them, the day in World War I when 27,000 people were killed and how Buffalo Bill made a profitable show out of the genocide of Native Americans.
President Emmanuel Macron was reported Monday to have selected last year's Goncourt winner, Leïla Slimani, as his representatives at the French-speaking nations group, la Francophonie.
Nazi on the run
The lesser-known prize, the Renaudot, went to Olivier Guez for La Disparition de Joseph Mengele (The disappearance of Josef Mengele).
The book follows Mengele, a doctor and Nazi party member who conducted notoriously cruel experiments on concentration camp prisoners, in his post-war flight to Argentina and Brazil.
Of his three years researching the book, Guez commented that he would not would not condemn anyone to "wake up every day thinking of Mengele", a "despicable personality of abyssal mediocrity".
He wrote the book because he wanted to examine "what life has in store for a man who had done what Mengele has done".