Keeping of one's vows is an act of worship. Allah has praised those who fulfill their vows with obedience to Him and by carrying out that which pleases Allah.
{They fulfill their vows and they fear a day whose harmfulness is alarming.} Al-Insaan: 7
The Prophet (sas) ordered us to fulfill our vows as long as they entail acts of obedience. This is a point of concensus among the scholars except for one opinion reported from Abu Hanifah which said that only those acts which are derived from something obligatory in Islam such as a vow to fast a certain number of days but not acts which are strictly voluntary such as I'tikaaf (isolation for worship in the masjid).
The Prophet (sas) said: "Whoever vowed to obey Allah should obey Him and whoever vowed to disobey Allah should not disobey Him." Al-Bukhari
Vows are a form of worship because if made to Allah and for an act of obedience to Allah fulfilling them becomes an obligation. Therefore, vows to other than Allah - such as those committed by some of the devotees of the graves dedicated to their "saints" in an effort to please them to get their needs met or for intercession - is shirk in worship without any doubt. This is because it involves glorification of the one to whom the vow is directed and because the fulfilling of vows is an act of worship which must be directly exclusively to Allah Most High. Vows to other than Allah are acts of disobedience and are null and void and there is no obligation of any kind to fulfill them.
The Prophet (sas) discouraged even the lawful types of vows because of the risk of being distracted from the omnipotence of Allah Most High. Abdallah ibn Umar narrates: "Once the Prophet (sas) began telling us not to make vows and he said: "It does not turn anything back, it only serves to extract some property from the greedy."
When one vows to Allah Most High, one has placed his hopes in Allah alone based on the knowledge that Allah alone is the Omnipotent (Al-Qadeer) and that whatever Allah wills is and whatever Allah does not will is not. That is why fulfilling oaths to Allah is obligatory. In contrast, one who vows to other than Allah has placed his hopes in other than Allah and divided his intention between Allah and between these concocted partners. Any act of worship when redirected to other than Allah or divided between Allah and between others is shirk against Allah.
Vows to other than Allah are falsehood for many reasons. Among them:
It is an act of worship and all acts of worship cannot be directed to any created being.
Vows to other than Allah are made to dead things and dead things possess no ability or power to act.
One who does this must believe that the dead one has some ability to act on their behalf or some actual effect in getting his needs met. If he didn't believe this, he would never direct his vows to that one. This belief is kufr.
He also fears the displeasure of this dead one should he fail to fulfill his vow. So he attributed to other than Allah the ability to bring benefit in the beginning and the ability to inflict harm should he fail to fulfill the oath. The shirk of this is clear.
Ibn Qudama, a great scholar of fiqh, said: "It is not valid to make votive offerings of candles, oil or such things to the places in which there are graves. The Prophet (sas) has cursed those who put in such places mosques or lamps." Al-Mughni: 2/383
Sometimes, the issue is confused by those who say about offerings to other than Allah: "The offering is to Allah and the reward is for the Shaikh." The Shari'a has allowed for a Muslim to give a charity on behalf of another, no doubt. And surely there is a possible benefit to the deceased from such giving on his behalf by the living or by their du'a to Allah on his behalf - as long as this is a pure act of giving not tainted with any false beliefs about causality and Allah's omnipotence. However, such is not the case with these offerings at the graves of righteous ones of the past: they go to make these offerings when in some difficulty seeking the intermediation or intercession of the dead one for their benefit and they fear the anger of this dead one should they not fulfill their oath. This is precisely the same as the vows of the first jahiliya which were made to the idols in the Ka'aba.
Nonetheless, an individual may be innocent - based on this claim of giving the offering solely for Allah and wishing the reward to the Shaikh - until the truth from the Qur'an and the Sunnah has been properly delivered to them.
We have just seen in the news this week a story of the second jahiliya of the Muslims. There is a dungeon where a Christian named St. George was allegedly chained up and tortured. Many Christians AND MUSLIMS (wa laa hawla wa laa quwwata illa billahi) come from all over Egypt to wrap themselves in these chains and seek miracles and aid in all that troubles them from the "blessings" in these chains. O Allah! Please do not punish us for the actions of the foolish among us!
Sacrificing - the ritual killing of animals - is one of the symbols of Islam and is an act of worship of Allah Most High. Allah said:
{So pray to your Lord and sacrifice.} Al-Kauthar: 3
Allah ordered his Prophet (sas) with these two acts of worship: prayer and sacrifice (nusuk). They are two of the best ways to worship Allah Most High. Prayer is the best act of the physical body in worship of Allah and sacrifice is one of the best economic acts of worship of Allah Most High. The Prophet (sas) and the Muslims did both of them for Allah alone while the mushrikeen of Makka worshipped the idols, called to them in du'a and sacrificed to them. Allah said:
{Say: Surely my prayer, my sacrifice, my life and my death are for Allah, Lord of the worlds.} Al-An'aam: 162
Ali ibn Abi Talib narrates: "Allah's Messenger spoke to me four words: Allah curses whoever sacrifices to other than Allah. Allah curses whoever curses their two parents. Allah curses whoever gives shelter to a criminal. Allah curses whoever changes markers on the land." Sahih Muslim
Sacrifice to other than Allah means to sacrifice for those who have died such as prophets, righteous men or others in hopes of their "blessings" or sacrifice to the Jinn or "spirits" in hopes of getting one's needs met or of keeping their harm away by appeasing them. All of this is major shirk and its perpetrator is deserving of the curse of Allah and His punishment.
Sacrificing can take many forms, summarized as follows:
To sacrifice for other than Allah in glorification of this other and seeking his pleasure. This is kufr and removes its doer from the fold of Islam.
To sacrifice without mentioning the name of Allah Most High. For example to kill an animal for food without mentioning anyone. This is haram and the meat is not lawful to eat.
To sacrifice seeking Allah's pleasure and mentioning His name over the sacrifice but intending it as a sadaqa on behalf of one who has died out of compassion for them and hoping that Allah's reward will reach them and benefit them because of this act. As long as this is not accompanied by any false beliefs about the deceased - such as any need for his intercession or that he can bring any harm or benefit in accordance to his pleasure/displeasure with the one sacrificing - is without doubt halal and is among the good deeds.
To sacrifice in the name of Allah and purely seeking to worship Allah - such as the sacrifices on the Hajj and on 'Eid al-Adh'haa. This is without doubt an act of worship of Allah Most High.
To sacrifice an animal just for the meat along with mentioning the name of Allah over it. This is one of the lawful acts which Allah has given us for our benefit.
An-Nawawi said (in his commentary on Sahih Muslim): "Sacrificing to other than Allah means to kill an animal in the name of other than Allah such as one who kills for an idol or the cross or Isa or to the Kaaba, etc. All of this is haram and the meat is not lawful regardless of whether the slaughterer is Muslim, Christian or Jewish. ... If he intended, along with that, glorification of the one in whose name he killed other than Allah and worship of him/it, the issue becomes kufr. If such a one was Muslim before this act, he becomes an apostate after it."