Is it permissible to estimate the value of a vow (slaughtering a fidaa' for one's son and sister's son) and buy meat with that amount to distribute to the needy, given the lack of experience in purchasing livestock and the absence of assistance?

The one who makes a vow must fulfill their vow according to what they committed to. Purchasing meat is not a substitute for slaughtering. Therefore, it is advisable to seek assistance from someone who can buy sheep, or from a trustworthy charitable organization. Inform them about the nature of the vow and provide them with the money to purchase the sheep and distribute them to the poor.