Translated from one of the sites Wikipedia links(the one with the bad SSL)
From godly episodes from the lives of saints to crucifixions and martyrs: visual representations opened up imaginary spaces into which the believers of the Middle Ages could empathize. Soul salvation and soul distress were thus internalized and integrated into one's own existence. St. Vitus, as a role model for Christian piety, renounces the worldly seduction of music-making virgins. According to the legend, Veit finally found his martyrdom in a cauldron with boiling oil - or in some cases in an oven, as staged here by the βMaster of the Vitus Legendβ.
So he was deep fried in an oven or a cauldron/kettle