If he fulfils his vow, is it permissible for him to eat from it?
Question

                If a man makes a vow, can he eat from it or not?.
Praise be to Allah.The basic principle 
      concerning vows is that if what is vowed is something that is permitted in 
      Islam, it should be given in the manner specified by the one who made the 
      vow. If he did not specify anything then it is a kind of charity and may 
      be given in any way in which charity is given, such as to the poor and 
      needy. 

    With regard to eating from it, if the custom in the country 
    where he lives is for the one who makes a vow of giving something edible to 
    eat from it, it is permissible for him to do so, based on the local 
    traditions and customs. Similarly, if he intended when he made the vow to 
    eat from it, then both custom and intention apply to the part that he eats, 
    and that is not part of that concerning which the vow was made. A fatwa was 
    issued by the Standing Committee concerning that, which states the 
    following: 
The way in which a vow is 
    to be fulfilled is to give in the manner intended by the one who made the 
    vow, within the limits of sharee’ah. So if he intended the meat mentioned in 
    his vow to be given to the poor, it is not permissible for him to eat from 
    it, but if he intended in his vow to give it to his family or to a group of 
    whom he is a member, then it is permissible for him to eat as one of them, 
    because the Prophet  (peace and blessings of Allaah be upon him) said: 
    “Actions are but by intentions and each person will have but that which he 
    intended.” The same applies if he stipulates that in his vow or if that is 
    the custom in his country. 

    From Fataawa al-Lajnah al-Daa’imah li’l-Ifta, 23/390 

    And it says in Fataawa al-Lajnah al-Daa’imah (23/392): 
    Whoever makes a vow to offer food, the basic principle is that the one who 
    makes the vow should not eat from it unless he has stipulated or intended to 
    eat from it, in which case it is permitted for him to eat as he stipulated 
    or intended. 

    And Allaah knows best.