The expiation for a vow is the same as the expiation for an oath

The vow: According to the Shariah, it is when a religiously accountable person voluntarily obligates himself to perform an act of worship for the sake of Allah Almighty.
The expiation for an oath is to feed ten poor people, clothe them, or free a slave. If one is unable to do any of these, then he must fast for three days.
The wisdom behind the expiation is that a Muslim should respect the vow, avoid breaking it, and not make vows carelessly or habitually.
Types of the vow: 1. Unspecified vow: For example, saying, "I vow to Allah if I am cured," and then saying nothing further or intending no specific act—in this case, upon being cured, he must offer the expiation of an oath. 2. Vow of obstinacy or anger: This is when one makes a conditional vow, intending to prevent or compel an action—for example, (If I speak to you, it is due upon me to fast for a month. Its ruling: the person has the choice between fulfilling what he vowed or expiating with the expiation of an oath if he did the act (speech). 3. Permissible vow: Such as "I vow to Allah that I will wear my garment." Its ruling: he may choose between wearing the garment or offering the expiation of an oath. 4. Disliked vow: Such as "I vow to Allah that I will divorce my wife." Its ruling: it is recommended that he expiate for it and not act upon the vow; if he acted upon it, no expiation is required. 5. Sinful vow: Such as "I vow to Allah that I will steal." Its ruling: it is forbidden to fulfill such a vow, and he must expiate with the expiation of an oath; if he fulfills it, he sins, but no expiation is due. 6. Vow of obedience: Such as "I vow to Allah that I will pray such-and-such," intending nearness to Allah. If he makes it conditional—for instance, upon the recovery of a sick person—he must fulfill it if the condition occurs. If it is unconditional, he must fulfill it unspecifically.