A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
num 30:1VOWS ARE NOT TO BE BROKEN. (Num. 30:1-16)
This is the thing which the Lord hath commanded--The subject of this chapter relates to vowing, which seems to have been an ancient usage, allowed by the law to remain, and by which some people declared their intention of offering some gift on the altar or abstaining from particular articles of meat or drink, of observing a private fast, or doing something to the honor or in the service of God, over and above what was authoritatively required. In Num 29:39, mention was made of "vows and freewill offerings," and it is probable, from the explanatory nature of the rules laid down in this chapter, that these were given for the removal of doubts and difficulties which conscientious persons had felt about their obligation to perform their vows in certain circumstances that had arisen.
num 30:2If a man vow a vow unto the Lord--A mere secret purpose of the mind was not enough to constitute a vow; it had to be actually expressed in words; and though a purely voluntary act, yet when once the vow was made, the performance of it, like that of every other promise, became an indispensable duty--all the more because, referring to a sacred thing, it could not be neglected without the guilt of prevarication and unfaithfulness to God.
he shall not break his word--literally, "profane his word"--render it vain and contemptible (Psa 55:20; Psa 89:34). But as it would frequently happen that parties would vow to do things which were neither good in themselves nor in their power to perform, the law ordained that their natural superiors should have the right of judging as to the propriety of those vows, with discretionary power to sanction or interdict their fulfilment. Parents were to determine in the case of their children, and husbands in that of their wives--being, however, allowed only a day for deliberation after the matter became known to them; and their judgment, if unfavorable, released the devotee from all obligation [Num 30:3-8].
num 30:3If a woman also vow a vow unto the Lord, and bind herself by a bond, being in her father's house in her youth--Girls only are specified; but minors of the other sex, who resided under the parental roof, were included, according to Jewish writers, who also consider the name "father" as comprehending all guardians of youth. We are also told that the age at which young people were deemed capable of vowing was thirteen for boys and twelve for girls. The judgment of a father or guardian on the vow of any under his charge might be given either by an expressed approval or by silence, which was to be construed as approval. But in the case of a husband who, after silence from day to day, should ultimately disapprove or hinder his wife's vow, the sin of non-performance was to be imputed to him and not to her [Num 30:15].
num 30:9every vow of a widow--In the case of a married woman, who, in the event of a separation from her husband, or of his death, returned, as was not uncommon, to her father's house, a doubt might have been entertained whether she was not, as before, subject to paternal jurisdiction and obliged to act with the paternal consent. The law ordained that the vow was binding if it had been made in her husband's lifetime, and he, on being made aware of it, had not interposed his veto [Num 30:10-11]; as, for instance, she might have vowed, when not a widow, that she would assign a portion of her income to pious and charitable uses, of which she might repent when actually a widow; but by this statute she was required to fulfil the obligation, provided her circumstances enabled her to redeem the pledge. The rules laid down must have been exceedingly useful for the prevention or cancelling of rash vows, as well as for giving a proper sanction to such as were legitimate in their nature, and made in a devout, reflecting spirit.