The difference is that Zachary questions that God would give him and Elizabeth the strength to bring forth a child, while Mary questions how the new command from God squares with an obligation to God she already has.
If I am God and I tell you, -- Fly off the cliff! -- and you say "er, no wings" -- you are being disobedient. But if I tell you -- Fly! Swim! -- you are entitled to ask how to do both tasks at the same time.
It's the Catholics who tell me that no one knows if they will persevere until the end. Yet you turn right around and say that Mary made a vow to God and knew that she would keep it for the rest of her days. How can that be if no one knows they will persevere? (I'm pinging jokus and forest keeper to this because I believe they were involve it this discussion as well.)
If such is the case as you suppose, and fitting in with the Catholic teaching that NO ONE knows if they will persevere, the natural question for Mary would have been, "...but I am a virgin, does this mean that I will break my vows?" Mary should have questioned her own commitment. It would have been persumptious and arrogant for her to say, "How can this be since I made a vow to God?"
Mary knew that she, by herself, would bring forth a child simply because the angel told her.