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, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Memorizing the Quran is one of the best and greatest acts of worship. When one makes a vow to obey Allaah, then he is required to fulfill it, unless he is unable to do so. In the latter case, he is required to offer an expiation similar to that of breaking an oath. It is permissible that one delays the fulfillment of a vow until he is able to do so. Immediate fulfillment of a vow is not due unless the formula of the vow implies this. In the latter case, he is required to offer an expiation, as we have stated earlier.
There is no relation between the non-fulfillment of a vow and sickness or other trials. Even if he is not liable to vows, a Muslim is afflicted with sickness and other trials. Everything a Muslim experiences of sickness or trials, whether that is in his body or any other thing, occurs according to the predestination of Allaah and His Divine Decree. One's sins are also one of the reasons behind trials. So, we advise our noble brother to be patient, ask for wellbeing and safety from Allaah regarding his religiousness as well as all his conditions in the worldly life and the hereafter, and intensify in giving charity and maintaining kinship ties.
The following was stated in Fat-h Al-Qadeer:
Ahmad, Ibn Rahawayh, Ibn Ma‘een, ‘Abd ibn Humayd, Al-Hakeem At-Tirmithi, Abu Ya‘la, Ibn Al-Munthir, Ibn Abi Hatim, Ibn Mardawayh, and Al-Hakim narrated that ‘Ali ibn Abi Talib said, "Shall I tell you about the best verse in the Book of Allaah that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, told us about? It is the verse where Allaah Almighty says (what means): {And whatever strikes you of disaster—it is for what your hands have earned; but He pardons much.} [Quran 42: 30] The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: "I will interpret it for you, O ‘Ali: It means that whatever strikes you of sickness, punishment or trial in this world, it is for what your hands have earned; and Allaah is too generous to punish you again in the Hereafter. Whatever Allaah pardons in this world, Allaah is too generous to punish you again after having forgiven you.'" ‘Abd ibn Humayd and At-Tirmithi also narrated on the authority of Abu Moosa Al-Ash‘ari that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said, "Never does a disaster strike a servant (of Allaah), whether it is grave or simple, except due to a sin; and Allaah pardons much more," then he recited the verse {And whatever strikes you of disaster…}
Allaah Knows best.