What a bizarre un scriptural extrapolation!
She was uttering a statement IN THE moment...that she had never Biblically "known" a man so was confused as to how she could be with child.
To make that into a vow is to change the meaning of scripture. Not a Christian thing to do.
There was no vow to violate.
Poor Mary, the things attributed to her that make no sense and actually show a great disrespect for her are legion unfortunately.
Mary’s question to the angel makes no sense unless she had made a vow of virginity. She was already married. If she expected to have relations with her husband in the near future, her question to the angel makes no sense.
Your assertion that Mary was speaking only of the PRESENT MOMENT has no basis in the text. The angel did NOT say that she WAS ALREADY pregnant.
http://www.catholic.com/magazine/articles/the-case-for-mary%E2%80%99s-perpetual-virginity