In the past month, I received an excellent e-mail question regarding the law, women, and application. The original response was unformatted but is republished here with edits along with the original question,

Several months ago we had a momentary conversation where you mentioned something about finding the heart of the matter in the Old Testament. (You used better words) But then that heart, or the intention can be brought forward into time. What is the heart of Numbers 30? That says that if a woman makes a vow without her husbands/fathers approval, if he nullifies it then it is nullified, but if he doesn't speak then it stands?

It seems like the coverture type of laws, where a woman was not legally capable of making a contract because her husband/father had the real control.

(The edited response)

This is a great question. Numbers 30 has become something of a litmus test for me in regards to an exagerated patriarchy and complementarianism — in my opinion of course. There are a couple ideas I would want to flesh out from the passage as a whole in order to tackle your question. As is to be expected, I'm going to draw heavily off of John Calvin to answer some of the contextal questions.
 
First, I would argue that God is primarily concerned in Numbers 30 with the binding nature of these covenants and promises. Thus the passage starts by saying “he must not break his word; he must do whatever he has promised” (Num 30:2). To put it in a protection from a negative, God’s concern is deception. Calvin summarizes as much when he lumps this passage under the Fourth Commandment in his commentary. So with this in mind, we should first think that everything in the chapter is case law of this principal point. Calvin — again — summarizes as much when he concludes his discussion on these verses by saying they were added “to prevent frauds.” Calvin certainly does not see this passage as fundamentally putting women below men (something he did definitively in 1 Corinthians 11). Instead, the passages prohibits the explotation of the social structure for fraud.
 
This leads to a “second” which is not truly a point but a speculative question and observation. How can we read these verses such that they help “prevent fraud?” Calvin depicats two practical cases that are directly prohibited by these case laws. The first is that a husband could not remain silent, and then at a later point contradict (read nulify) his wife’s vow or contract. This could be used by a married couple to deceive individuals (eq. the wife makes a vow/promise and before the time of fulfillment the husband annuls it). Similarly, a women moved out from under her father or husband — through marriage, divorce, or death — could not then nullify a promise. She was not left off the hook by a timely divorce or marriage. The vow ultimately lay with her. 

These attempts to “get out of a vow” seem to be what Jesus was referring to in the New Testament (Matthew 5:33-37). Christ’s point there is incorrectly simplified by some to “don’t make vows,” but this has been shown historically to miss the point. Christ is further removing the litnany of excuses for getting out of the vow. He is expanding the conviction and spirit of this Old Testament case law in Numbers 30. His summary is let “yes” and “no” be enough. So I would argue that when Jesus looks back upon these verses His emphasis is on being honest and truthful.
 
Third, all of this seems to be confirmed by the final case law provided — widows (Num 30:9). Women could make vows without men. And they were obligated to fulfill them. It does not seem to me that there is a blanket “not legally capable of making a contract” principal that undergirds this particular case law or the rest of the passage.
 
To summarize, it seems to me that the text is concerned with condemning many possibly method of fraudulent behavior among Israel. Because of the social structure, the case law for men and widows/divorced is simple — their word is their bond. But in the case of women in the house of a father/husband, the case law goes one step further to make sure that neither the woman or man can use the social structure for fraudulent behavior. God’s law is gracious in this regard. In today’s society this law would apply to shady bank accounting or commercial enterprising by spouses. But alas, we do not have holy and just laws.
 
With more study I could probably provide a better answer. I hope this helps.