Mary was chosen by God, but sometimes even those chosen by God go astray. We see that in David, Solomon, Abraham, etc. The list goes on.
Did their chosenness negate their free will in choosing to do what God told them they should do? No.
So why is Mary any different? She had the exact same choice that Eve had - she could choose to go through with it, or she could choose otherwise. But - God left it up to her. She could have chosen otherwise, and God would have left her to her graces.
This is why the Catholic church teaches that man was saved through the obedience of Mary whereas man was condemned through the disobedience of Eve. Mary chose to bear Christ.
And Mary is not a vessel - she is a woman a person. Christ is truly her son just as any other child!
Are you saying that if Mary had rejected the "choice" to bear the Messiah, she would have remained sinless? Would she then have gone to heaven anyway because she didn't need a savior? If so, would Christ's redemption not be needed for her since she never sinned because of the graces she had already been given of God? If that is the case, as you are sounding like it is, then why couldn't/wouldn't God have just done that for ALL mankind and EVERYONE would go to heaven and Jesus wouldn't have needed to die?