Albert Barnes Commentary Numbers 30:2

"When a man voweth a vow unto Jehovah, or sweareth an oath to bind his soul with a bond, he shall not break his word; he shall do according to all that proceedeth out of his mouth." — Numbers 30:2 (ASV)

The “vow” was positive; the “bond” negative or restrictive. By a vow a man engaged to dedicate something to God, or to accomplish some work for Him: by a bond he prohibited himself from some privilege or enjoyment. A vow involved an obligation to do: a bond, an obligation to refrain from doing.