A vow is only as good as the character of the person making it, and valid only for as long as the situation remains unchanged. In other words, vows are pretty much worthless, irrespective of the religiosity or irreligious nature of the people making these forever-and-ever professions. That's not to say that marriages never last — clearly many do and can be quite fulfilling — but that's not predicated on promises made about a future unknown. People need to grow together over many years to have that sort of relationship longevity, but it's not possible to know one's own growth potential let alone that of another person, so it's always a roll of the dice no matter how pure the intent on the wedding day.