לרפואת הרב זאב חיים בן חי' אידל בתוך שח"י
The idea behind taking a neder [vow] is that we all experience moments of inspiration and we want to hold on to them. Otherwise, they just disappear like a passing dream. So at that moment of inspiration we take upon ourselves something in order to preserve that special moment and to make it last. A person FINALLY gets engaged after 26 YEARS [of singlehood - so he is only 26] but he is SOOOO thnankful to Hashem that he takes upon himself to learn all of seder nashim by his first anniversary. Gemara. Bavli and Yerushalmi!! THEN it lasts.
But sometimes a neder is made out of anger or or other impure motivation. For that we have the mechanism of חרטה - regret. He goes to the chochom and expresses his חרטה and is freed from the neder.
At other times his intentions were pure. Just the circumstances don't allow the person to keep his vow. When he wasn't fully aware of the reality of the circumstances at the time of the neder, he may be released by a chochom through the means of a פתח.