Is it permissible to postpone the monthly vow payment to later dates, and is it permissible to help the unmarried brother marry from it, and is it permissible to send the mother for Hajj from it?

A Muslim must fulfill a vow of proximity (nadhir al-qurbah) in the manner in which it was vowed. This vow is valid and must be fulfilled. It appears that there is no harm in delaying the payment of the mentioned percentage beyond the time the salary is received, if there is a need, because the general principle for fulfilling vows is that it can be done at leisure (on tarakhi), unless its wording or intention necessitates immediacy. So, if you did not intend to pay it immediately, there is no harm in delaying. However, if you intended to pay it upon receiving the salary, then delay is not permissible. If you did not specify a recipient for the vow, you may help your brother with his marriage if he is in need, or send your mother for Hajj if she needs it. However, it is not permissible to spend it on your mother's obligatory maintenance and clothing, which are already your responsibility.