After completing his studies, he went on a grand tour, travelling in Britain and France, where he collected material for his first publication, the Venatio novantiqua (1645), an edition of Latin poetry on the subject of hunting.
This was not a popular field of study at the time, the historical languages deemed most worthy of academic attention being Greek, Latin, and Hebrew.
This shift in interests led to some tension between Van Vliet and several of his old friends, who were not best pleased to see a man they esteemed as a Latinist turn to study lesser things; a letter survives from Nicholas Heinsius, who had been a fellow student at Leiden, addressed "to Vlitius, the authority on antiquities both barbarian and scholarly",[1] a veiled criticism which appears not to have gone unnoticed.
Despite this tacit disapproval, however, Van Vliet began to study ancient books and manuscripts in various Germanic languages, including English.
It was in 1659 that he began to correspond regularly with his more famous contemporary Franciscus Junius, who was then resident in England, but visiting the Netherlands frequently.