In 1625, he was the preacher/priest at the church of the "Panagia Trimartyri", in Candia, Crete. Later, Vlastos was one of numerous famous teachers who taught at the school of the Sinaitic Metochion of the St. Catherine's Monastery (During the Venetian occupation, the Monastery housed a school). Other teachers were Ioannis Morezenos, Iosif Doreianos, Ieremias Palladas. Among his numerous famous pupils was Constantinos Loukaris (Religious name: Kyrillos Loukaris) who later did become firstly the Orthodox Patriarch of Alexandria and later the Ecumenical Patriarch in Constantinople. Another student in the same school was the future patriarch Meletios Pegas.
Meletios Pegas and Maximos Margounios were friends of Vlastos and he kept in touch by correspondence with them.