Had Mary not been a willing vessel God would have chosen someone who was. Do you think that God would have not fulfilled His plan of salvation if Mary had been unwilling?
Yes, but if you believe in the hypostatic union (just a fancy way of saying that Christ was fully God and fully man at the same time), and you believe the Scriptural geneology that is included in the Gospels, then you should believe that Jesus "took His flesh" of the Virgin Mary. In a more "scientific" way, Mary's ova, which only had half the chromosones needed for a new human life, was "made complete," through the power of the Holy Ghost, and at that moment, Jesus was conceived. If you believe that Mary was "just a vessel," then you are basically saying that Jesus wasn't a true member of God's Chosen People, because you need to be truly descended from a Jewish mother to be Jewish! The Messiah had to be born from the descendants of Abraham, Issac, and Jacob, otherwise the Scripture passage that says "There shall come forth a rod out of the root of Jesse... and the Spirit of the Lord shall rest upon Him" (Isaiah 11), along with many other prophecies concerning the Messiah, would be lying.
In conclusion, if the Messiah needed to be truly descended from a Jewish mother to be Jewish, and this Jewish mother had the stain of original sin upon her (since she would be descended of Adam), then that stain would have to be passed on to Jesus. There's no escaping that if you believe He is true Man, and is descended of Abraham. The Father, in His great Wisdom and sense, knew what needed to be done in order for His Son to be born. That is why Mary had to preserved from original sin.