Forgot this bit. This is not just poor Christology, it is poor human biology. However, in context of first century beliefs and medicine, it is understandable.
The ancient Greeks (and by extension everyone else around who was listening, because they were very influential) believed that the human child was contained in the sperm, which was a miniature humunculous. The man planted this humunculous into the woman, who was no more related to the child than a field is related to the vegetable planted in it; that is, it is essential to provide the basis for growth, but does not contribute to its identity.
Along with this thinking, the Greeks believed male children to be perfect, and female children to be an abberation or to be misbegotten in some way. In fact, our own knowledge of genetics shows that it is the other way round.
Clearly, ancient Jewish thought did not concur with this, but wherever we find in Scripture any mention of any woman being a vessel, this is a contamination from Greek thought, and it is wrong.
As we know, the man and woman contribute equally to any child, and to call any woman on earth 'merely a vessel' to her child is a gross insult. To say it of Our Lady is far worse.