In 1584, he became counsellor of the parlement of Paris, and as deputy for Paris to the Estates of the League he pronounced his most famous politico-legal discourse, an argument nominally for the Salic law, but in reality directed against the alienation of the crown of France to the Spanish infanta, which was advocated by the extreme Leaguers.

King Henry IV of France acknowledged his services by entrusting him with a special commission as magistrate at Marseille, and made him Master of Requests.

[1] In 1595, Vair published his treatise De l'éloquence française et des raisons pour quoi elle est demeurée si basse, in which he criticizes the orators of his day, adding examples from the speeches of ancient orators, in translations which reproduce the spirit of the originals.

Ferdinand Brunetière points out the analogy of Du Vair's position with that afterwards developed by Blaise Pascal, and sees in him the ancestor of Jansenism.

The reformer of French poetry learned much from the treatise De l'éloquence française, to which the counsels of his friend were no doubt added.