A case can be argued that if a man makes a reckless vow with his mouth - saying I will rob the bank tomorrow - then I believe it could be compared to something like coveting a neighbor's wife. You haven't done the physical act. But as Christ says you've already committed adultery in your heart.
But if you made a vow to do something sinful, you do NOT have to keep that vow.
Change my mind.
It's really quite simple. Taking the "vow to rob a bank" example, you now have two options:
1) Rob the bank. This is theft, which is a sin.
2) Don't rob the bank. This is breaking a vow, which is a sin.
Both options are sinful. You cannot choose not to sin - whatever you do will be a sin. But also that sin will be forgiven by God when you repent. So now you simply need to choose the lesser sin, the one that is least harmful the one with the least real-world negative consequences. Obviously, breaking the vow is by far the least harmful. So don't rob the bank, and ask God to forgive you both for breaking your vow and for making such a foolish vow in the first place.
What about conflicting vows? If a man did the "forsaking all others" vow but a few years later made a vow to another woman. Which vow would you say is invalid?
Here's another scenario what if a man married a woman using the traditional vows then a few years later he married another woman using the "forsaking all others" which one would be invalid?
Again, this is very simple, but you're asking the wrong question. As
@Mark C said, the question is not "which vow is invalid". Forget about the idea of vows being valid or invalid. A vow is a vow. Both are valid.
If the man vowed to his first wife to forsake all others, she did not release him from this vow, and he decided anyway to go ahead and marry a second woman, then he has sinned by breaking his vow.
However, he now does have a second wife. He must not sin further by divorcing her - two wrongs does not make a right. He must choose to
stop sinning, not keep adding more sin trying to fix things by digging himself into an even deeper hole.
So he now has two wives, like it or not.
Same goes for the man who has one wife, with monogamous vows, then has a secret affair. He is now required by scripture to marry the second woman. He can't get out of this by saying "but I vowed to be monogamous". Sure he vowed that - but he's already broken that vow. He should have thought of that before he took his pants off. It's too late now, the vow has been broken and that cannot be changed. Now he has two women anyway and he's going to have to man up to the consequences.
