Is the failure to achieve what the one who made the vow sought and requested a sufficient reason to justify going back on the vow and not fulfilling it, even if one is capable of doing so?

The question contained two inquiries: Is it permissible to retract a vow? And if one retracts it and their need is fulfilled, is it then obligatory to fulfill the vow? Both lead to the same meaning: Can one retract a vow, and does this retraction become void if the need is later fulfilled, or not?