All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Since you did not specify any beneficiary for the charity in your vow, there is no harm if you give the vowed money to your friend so that he can buy the essentials for his marital home. You are not required to tell him about the nature of the money that you give to him.
Moreover, it is permissible to give your friend some of the Zakah money to buy what he needs for his marriage, provided that he does not spend it with extravagance.
Nevertheless, it is impermissible to give a sum of money for the fulfillment of your vow and Zakah simultaneously. This is because the Zakah and vows are two separate religious obligations. Offering a sum of money for both of them does not clear you from the obligation of paying them. We implore Allaah, the Exalted, to grant us success and guide us to what pleases Him and that which He loves.
Allaah Knows best.