A vow is a promise that is made, and here refers to promises made to the Lord.
V.3 A woman makes a promise or vow to the Lord. Sometimes we make promises that we are not able to carry out. Or perhaps we wish we had never made that promise. When a woman made a vow, she had to keep her word. No matter how difficult it might be, she could not go back on her word.
V.8 The remarkable provision. If her husband or her father (v. 5) heard her make a vow and he knew that it would be very difficult for her to keep that vow, then he could “disallow” that vow (vvs. 13-15). Her husband had the right to let her bear the full responsibility of her vow, or if she didn’t he must himself. Then she is free from the responsibility.
V.15 “Then he shall bear her iniquity.” What a lesson there is in all this! At the crucifixion of our Lord Jesus, the leaders of Israel cried out to Pilate, “His blood be on us and on our children.” What a terrible vow! They said we know what we are doing, we want this Man to be put to death and we will take the full guilt of it to ourselves and to our children. If that vow stood, then there could be no blessing for that nation forever. They would always be held guilty of the murder of the Lord Jesus, and God Himself could not bless them. But the Lord Jesus heard that terrible vow — He really was like the husband of Israel. Though He was the rejected and hated One, He did not want that terrible vow to stand. So He said, “Father forgive them, for they know not what they do.” He disallowed (canceled) that vow! What wonderful love! He wanted them to be blessed and forgiven and took the guilt upon Himself instead. And now God is free to bless His people Israel, and He will do it after we believers are taken to be with the Lord.