In 1924, Piet was one of the founders of a Flemish study group at the Université libre de Bruxelles.
In 1938, together with Henri Storck and André Thirifays, he founded the Cinémathèque royale de Belgique.
Notwithstanding what German occupiers had done to his father, he vehemently protested the execution of Flemish collaborationist August Borms.
He again became a minister for Internal affairs in 1954 and for four years had to defend the secularist school policies of the Liberal-Socialist coalition under Prime Minister Achille Van Acker in the face of Roman Catholic opposition, at one time controversially forbidding Belgian Radio to report on a large-scale demonstration against the new school laws proposed by Education minister Leo Collard.
When armies of the Warsaw pact invaded Czechoslovakia in August 1968, Vermeylen, who was secretly visiting Brno as a simple tourist, barely managed to escape to Austria.