According to dictionary in the NKJ Bible, Vine's Expository Reference Edition,
A. Verb nadar "to vow," i.e., to make a promise to another, with sanctions for not completing the promise.
B. Noun neder, "vow; votive offering."
The vow has two basic forms, the unconditional and the conditional.
The unconditional is an "oath" where someone binds himself without expecting anything in return, Psalm 116:14.
The conditional "vow" generally had a preceding clause before the oath giving the conditions which had to come to pass before the "vow" became valid, Genesis 28:20-22.
Numbers 30 deals with the law concerning vows.
In reference to Acts 18:18 (NKJV) which states
So Paul still remained a good while. Then he took leave of the brethren and sailed for Syria, and Priscilla and Aquila were with him. He had his hair cut off at Cenchrea, for he had taken a vow.
The Life Application Study Bible, KJV, gives the following explanation for this verse:
This vow Paul took was probably a temporary Nazarite vow that ended with shaving of the head and offering the hair as a sarifice (Numbers 6:18).
Numbers 6:18 (NKJV) reads;
Then the Nazirite shall shave his consecrated head at the door of the tabernacle of meeting, and shall take the hair from his consecrated head and put it on the fire which is under the sacrifice of the peace offering.
However, the Old Testament allows for "redeeming" the "vow"; by payment of an equal amount in silver, a person, a field, or a house dedicated by "vow" to the Lord could be redeemed, Leviticus 27:1-25.
This practice declined in Jesus' time. The "votive offering," Ezra 7:16, is also a kind of thank offering, Nahum 1:15. Here even Gentiles expressed their thanks to God presumably with a gift promised upon condition of deliverance, cf. Numbers 21:1-3.
If one chooses to take a vow before God to promise to "love and cherish" during a wedding ceremony, for instance, this is not a conditional vow. One isn't asking for something in return, but simply making a promise before and to God. These types of vows do have a place these days.
However, Jesus warns against oaths in Matthew 5:33-36.
Conditional vows and/or oaths, those vows made expecting something in return would seem contrary to a true heart of worship and love for God. My belief is that God does not want conditional praise, conditional worship, conditional love or conditional promises.