This is the first verse cited in defense of the IC...though the Catholic Encyclopedia admits "No direct or categorical and stringent proof of the dogma can be brought forward from Scripture."
It further admits the Vulgate translation is in error: The translation "she" of the Vulgate is interpretative; it originated after the fourth century, and cannot be defended critically.
I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel. Gen 3:15 Douay Rheims
And I will put enmity Between you and the woman, And between your seed and her seed; He shall bruise you on the head, And you shall bruise him on the heel." Gen 3:15 NASB
The doctrine of the Immaculate Conception is not dependent on that particular reading of Scripture.
It further admits the Vulgate translation is in error
All translations contain errors, even the KJV. The translation that you give from the NASB, "He shall bruise…", could also be argued to be erroneous if the pronoun is taken as referring to the "seed" as all the descendants. In any case, all modern Catholic translations, including the Nova Vulgata, render it as either "he, "it," or "they." Why belabor the point of what is now an historical translation? But I do notice the common tactic of changing the subject when a difficulty to the Protestant case is shown.
To return to the original issue, this minor translation controversy pales in comparison to the dishonesty of Luther adding "alone" to a verse in order to force his theological point of view. Paul in Romans was speaking against the Judaizers' insistence on adhering to Mosaic Law and not contrasting faith against following the moral law. "Faith alone" finds no support in Romans.
I actually like this translation better than "Her offspring....."