In sum, it seems likely that the story of Veronica is a delightful legend without any solid historical basis; that Veronica is a purely fictitious, not a historical character, and that the story was invented to explain the relic. It aroused great interest in the later Middle Ages in the general devotional context of increased concern with the humanity of Christ, especially the Holy Face, and the physical elements of his Passion. [David Hugh Farmer, "The Oxford Dictionary of Saints," 1978]
Hence vernicle (mid-14c.) "picture of the face of Christ," from Old French veronicle, variant of veronique.