What happens if you try to break an unbreakable vow? Do you die, lose your powers?
And if you vow to do something, and you just haven't done it yet, when does that become breaking the vow? If Snape had passed up the opportunity to do what he did on the tower, that wouldn't necessarily be breaking the vow because maybe he was planning to do it later?
The penalty for breaking an unbreakable vow is death. I think Snape would have had serious problems if he had not come to Draco's assistence there and then on the tower. "But I would have killed him later, honest" - I don't think so.
Do you think the punishment just happens automatically?
Or is there some kind of godlike judge who follows up? Or the Bond maker or person to whom you made the vow?
What if you make an unkeepable vow -- I promise to find Saddam's weapons of mass destruction?