St. Jerome's Vulagte was based on Hebrew texts but not the Masoretic text as St. Jerome translated in the 5th century the Masoretic text dates from the 10th century.
But as for stating that portraying the Theotokos as treading on the serpent is an error is a bit much. It is devotional hyperbole, of which we Eastern Christians are guilty as well.
From the Akathist:
Having shed the light of truth in Egypt, You expelled the darkness of falsehood; and unable to bear Your strength, O Saviour, her idols fell; and they that were set free from them cried to the Theotokos:
Rejoice, Uplifting of men. Rejoice, Downfall of demons.
Rejoice, you who trampled upon the delusion of error. Rejoice, you who censured the deceit of the idols.
Rejoice, Sea which drowned the symbolic Pharaoh. Rejoice, Rock which refreshed those thirsting for life.
Rejoice, Pillar of fire, guiding those in darkness. Rejoice, Protection of the world, more spacious than a cloud.
Rejoice, Nourishment, successor to manna. Rejoice, Minister of holy joy.
Rejoice, Land of promise. Rejoice, you from whom flows milk and honey.
Rejoice, O Unwedded Bride.
Fr. Deacon Lance