What is required of one who is unable to fulfill his vow

Question:
I am a young man of twenty-six years. Allah decreed upon me one of the trials of this world that befall every young man: I am married, and I have three children living under my care after Allah, and my mother is advanced in age. Divine destiny prevented me from seeing them for approximately one year and six months. So I vowed to Allah that when I return to my home and my children—who, after my absence, came under the care of the charitable—that I would fast six days for Allah the Exalted, slaughter two sacrificial animals for Allah the Exalted, and visit Makkah and al-Madinah with my father, carry my mother on my shoulders, and perform the tawaf and sa'y with her.
When that trial lifted—due to the poor financial condition of myself and my family—I slaughtered one sacrificial animal and was unable to bring the other. Likewise, I was unable to take my family or my mother to Makkah and al-Madinah to fulfill my vow, due to my poor financial condition, to the point that I could not even perform the fasting.
Out of fear of falling into sin and burden, I sent my message to find a solution that pleases Allah.

Answer:
Praise be to Allah who facilitated your reunion with your parents and children, and we ask Him, Exalted and Majestic, to rectify the condition of you all, and to help you with what He loves and is pleased with.
As for the vow, it is obligatory for you to fulfill it according to your ability. Allah praised the believers who fulfill their vows in His saying: [Al-Insan:7].
And the Prophet (peace and blessings be upon him) said: It was narrated by the Imam al-Bukhari in his Sahih from Aisha (may Allah be pleased with her). So you must perform the second sacrifice when you are able, due to His saying, Exalted is He: [Al-Baqarah:286], and His saying: : [Al-Taghabun:16].
So whenever you are able and it becomes easy for you to buy the second sacrificial animal, do so and slaughter it, and give it in charity to the poor, unless you intended to eat it with your family, in which case you are to follow your intention; due to the saying of the Prophet (peace and blessings be upon him): I Agreed upon by al-Bukhari and Muslim. But if you vowed the slaughter and did not intend to eat it with your family, then you give it to the poor.
And it is obligatory for you to fast six days, because it is obedience to Allah, so you must fast them whenever you are able, even if they are scattered, unless you intended to fast them consecutively, in which case you are to follow your intention; due to the saying of the Prophet (peace and blessings be upon him): . So if you intended to fast them consecutively, fast them consecutively.
And it is obligatory for you, as well, to perform Hajj with your parents to Makkah and al-Madinah, as you vowed. If you intended 'Umrah, then it is 'Umrah, and if you intended Hajj, then it is Hajj, according to your intention, whenever you are able. Allah does not burden a soul beyond its capacity, and He says, Exalted is He: [Al-Taghabun:16].
And it is obligatory for you to take them to al-Madinah as well, because traveling to al-Madinah for prayer in the Prophet's Mosque (peace and blessings be upon him) is a recommended practice and a nearness to Allah. If you visit al-Madinah, greet the Messenger (peace and blessings be upon him) and his two companions, and this is the best for you. Visiting his grave (peace and blessings be upon him) and the graves of his two companions is prescribed for those who are in al-Madinah, and likewise for men who travel to it. What is forbidden is traveling solely for the purpose of visiting his grave (peace and blessings be upon him) only. But traveling for the mosque, and the visit is included in that, so there is no harm in it. And greet the Prophet (peace and blessings be upon him) and his two companions (may Allah be pleased with them both). As for women, they do not visit graves. But you and your father, and the men with you, as for women, they do not visit graves, but they pray in the Messenger's Mosque (peace and blessings be upon him) and send blessings upon him in the mosque, in the homes, and on the road.
It is prescribed for you and the men with you to visit al-Baqi' and visit the martyrs. All of this is prescribed for men. It is also recommended for you and the men and women with you to visit the Mosque of Quba and pray in it, because it is a noble mosque. Visiting it and praying in it is recommended for those who are in al-Madinah and for those who travel to it.
As for carrying your mother or your father when you perform Hajj with them during the tawaf and sa'y, there is no blame on you in that, if they are unable to walk during the tawaf and sa'y, and you are able to do so. But if they are able, then they must perform the tawaf and sa'y themselves, and there is no harm in them performing the sa'y while riding, like the other pilgrims and those performing 'Umrah. The matter in this is broad, and praise be to Allah.
As for carrying them, it is not obligatory for you to carry them, due to the hardship involved, and the lack of evidence for its prescription.
And it is obligatory for you to expiate this vow with the expiation of an oath, if you do not carry them. It is: feeding ten poor people, or clothing them. You give each one half a sa' of dates, wheat, or rice, or you clothe each one with a garment that suffices them for prayer, such as a shirt or an izar and a rida. It is not obligatory for you to carry them, but they perform the tawaf and sa'y themselves, as mentioned before, if they are able. And if they are unable, they are carried for the tawaf and sa'y, and praise be to Allah.
And we ask Allah to help you fulfill your vow, and to accept from us and from you, and from the rest of the Muslims. We advise you not to make vows in the future, because the Messenger (peace and blessings be upon him) said: . So we advise you in the future never to make a vow. Whenever a blessing comes to you, thank Allah for it, obey Him, and praise Him, and there is no need for a vow.
You said in your question: "But the destinies prevented me." It is better to say in such a case: "But Allah's decree was such and such." This is because destinies have no power of action; only Allah alone has the power of action. So you say in such a case: "Allah decreed upon me such and such," or "Allah willed such and such," thereby attributing the matter to Allah . And success is from Allah.