The vow does not bring anything near to the son of Adam which Allah has not ordained for him, but (at times) the vow coincides with Destiny, and this is how something is extracted from the miserly person, which that miser was not willing to give. (Using translation from Muslim 1640c)
"إِنَّ النذرَ لا يُقَرِّبُ من ابن آدم شيئًا لم يكن اللَّهُ قَدَّرَهُ له، ولكنَّ النذرَ يوافقُ القدرَ، فَيخُرْج ذلك من البخيلِ ما لم يكن البخيلُ يريدُ أَنْ يُخْرِجَ".