Is Vowing Part of Repentance?
Question

                Do we have any chance of being forgiven if we do not vow to never repeat the sin again?

I'm asking this because, if you think that you will most probably do it again, is there a point in vowing not to repeat the haram (forbidden) action (knowing that to break a promise to Allah constitutes fasting for 3 days)?
Praise be to Allah.
    
    No sin is too great for Allah to forgive, no matter what this sin is, so 
    long as the person has repented from it. Allah says (interpretation of the 
    meaning): 

    
    “Say: O ‘Ibadi (My slaves) who have transgressed against themselves (by 
    committing evil deeds and sins)! Despair not of the Mercy of Allah, verily, 
    Allah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful.” 
    [39:53] 

    
    The door of repentance is open so long as death has not come to a person, 
    and so long as the sun has not begun to rise from the west. 

    
    It was narrated that Ibn ‘Umar (may Allah be pleased with him) said: The 
    Messenger of Allah (peace and blessings of Allah be upon him) said: “Allah 
    accepts the repentance of His slave so long as the death rattle has not yet 
    reached his throat.” [Ahmad, al-Tirmithi]

    
    It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The 
    Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever 
    repents before the sun rises from the west, Allah will accept his 
    repentance.” [Muslim]

    
    So the Muslim should make the most of this opportunity and this great bounty 
    from Allah, and hasten to repent so long as there is still time; he should 
    not delay his repentance. 

    
    But this repentance has to be sincere, as Allah says (interpretation of the 
    meaning): 

    
    “O you who believe! Turn to Allah with sincere repentance! It may be that 
    your Lord will expiate from you your sins, and admit you into Gardens under 
    which rivers flow (Paradise) the Day that Allah will not disgrace the 
    Prophet (Muhammad) and those who believe with him. Their Light will run 
    forward before them and (with their Records, Books of deeds) in their right 
    hands. They will say: ‘Our Lord! Keep perfect our Light for us [and do not 
    put it off till we cross over the sirat (a slippery bridge over Hell) 
    safely] and grant us forgiveness. Verily, You are Able to do all things.’” 
    [66:8] 

    
    The scholars have stated that sincere repentance is that which meets five 
    conditions: 

    
    1-     It should be sincerely for the sake of Allah.

    
    2-     There should be real regret for sins committed in the past, and the 
    person should wish that he had never done them.

    
    3-     He should give up the sin immediately. If the sin consists of doing 
    something haram (unlawful), he should stop doing it at once. If the sin 
    consists of not doing something that is obligatory, he should start to do it 
    at once. If the sin has to do with people’s dues, his repentance is not 
    valid unless he gives them those dues.

    
    4-     He should resolve never to go back to that sin in the future.

    
    5-     That it should not come after the time when repentance is accepted[U1] , 
    as stated above.

    
    See Majalis Shahr Ramadhan by Ibn ‘Uthaymeen. 

    
    Hence we know that it is not one of the conditions of repentance that one 
    should not repeat the sin, rather the condition is the sincere resolve not 
    to repeat it. If a person repents from a sin that he used to commit, then 
    the shaytaan (devil) toys with him and makes him go back to it – there is no 
    power and no strength except with Allah – he should not despair of the mercy 
    of Allah. He should repent once again and Allah will accept it, for Allah’s 
    bounty and forgiveness are immense.  

    
    It was narrated from Abu Moosa al-Ash’ari (may Allah be pleased with him) 
    that the Prophet (peace and blessings of Allah be upon him) said: “Allah 
    spreads out His hand at night to accept the repentance of those who 
    committed sin during the day, and He spreads out His hand during the day to 
    accept the repentance of those who committed sin during the night (and this 
    will continue) until the sun rises from the west.” [Muslim] 

    
    It was narrated from Abu Hurayrah (may Allah be pleased with him) that he 
    heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 
    “A person committed a sin and said, ‘O Lord, I have sinned; forgive me.’ His 
    Lord said: ‘Does My slave know that he has a Lord Who forgives sin and 
    punishes for it? I have forgiven My slave.’  Then as much time as Allah 
    willed passed, and he committed the sin again. He said, ‘O Lord, I have 
    sinned; forgive me.’ His Lord said: ‘Does My slave know that he has a Lord 
    Who forgives sin and punishes for it? I have forgiven My slave.’  …” 

    
    [al-Bukhari, Muslim] 

    
    But the wise Muslim should be sincere towards Allah in his repentance, and 
    sincerely resolve not to repeat the sin, and regret what he has done in the 
    past. His repentance should not merely be words that are spoken by his lips 
    when the rest of his faculties pay no heed; that is the repentance of 
    liars. 

    
    With regard to making vows not to commit this sin again, there is no need 
    for that. Sincere repentance is that which fulfils the conditions mentioned 
    above. 

    
    It remains to comment on that which the questioner mentioned about the one 
    who breaks his vow having to fast for three days. This is not always the 
    case. Rather what the one who breaks his vow has to do is to free a slave, 
    or feed or clothe ten poor persons. If he cannot do any of these things, 
    then he has to fast for three days, but fasting is only an option if he is 
    unable to do any of the three other things, as is indicated by the verse in 
    which Allah says (interpretation of the meaning): 

    
    “Allah will not punish you for what is unintentional in your oaths, but He 
    will punish you for your deliberate oaths; for its expiation (a deliberate 
    oath) feed ten Masakeen (poor persons), on a scale of the average of that 
    with which you feed your own families, or clothe them or manumit a slave. 
    But whosoever cannot afford (that), then he should fast for three days. That 
    is the expiation for the oaths when you have sworn. And protect your oaths 
    (i.e. do not swear much). Thus Allah makes clear to you His Ayat (proofs, 
    evidences, verses, lessons, signs, revelations, etc.) that you may be 
    grateful.” [5:89] 

    
    And Allah knows best. May Allah send blessings and peace upon our Prophet 
    Muhammad and his family and Companions.