So here’s the quote from Saint Jerome’s Hebrew Questions on Genesis, trans. C.T.R. Hayward (Oxford: Clarendon Press, 1995), 33.
“15. * It shall watch your head, and you shall watch its heel*. More correctly it has in the Hebrew it will crush your head, and you shall crush its heel. For our footsteps are indeed shackled by the serpent, and the Lord shall crush Satan under his feet swiftly.”
To review, the Latin text of the Nova Vulgata is “inimicitias ponam inter te et mulierem et semen tuum et semen illius ipsa conteret caput tuum et tu insidiaberis calcaneo eius”
Jerome seems concerned to point out that the form of insidior(?) used in his translation is not part of Hebrew text. Otherwise he observes no other differences or problems.
For purposes of this discussion, Jerome’s rendering of the Hebrew text follows logically with the wording found in MT. He renders the subject of the verb “to crush” in the Hebrew Text as “it”–here it would be useful to have the Latin original–simply because English has ungendered pronouns. But no matter. What seems clear that Jerome reads “it” to be the antecedent (masculine in Hebrew) seed from the previous clause, which again follows the MT quite smoothly. Hayward’s rendering of the subject as “it” also follows smoothly, considering that “semen” is neuter (“it”) in Latin. “It” is the seed that crushes. Interpretively Jerome understands the seed to be “the Lord” based on his comments which follows, not the woman, or interpretively, the mother of the Lord.
Further, Jerome notes no oddities or difficulties with the Hebrew text (such as a perceived subject/verb disagreement).