The Israelites were not told to make vows, but if they voluntarily made them, God said they must conscientiously perform them. Man is ever ready to boast of his strength, not being conscious of his own weakness. Israel, on hearing the law, did not hesitate to say, "all that the Lord hath said we will do;" but they alas, miserably failed. The law made vows binding, and gave directions as to exceptional cases where it was impossible to perform them. Numbers 30:2-14; Deuteronomy 23:21-23; Psalm 1:14; Ecclesiastes 5:4,5; Nahum 1:15; etc.
The only instances of vows in the N.T. are those of Paul (or Aquila, as some judge) at Cenchrea, which is shrouded in mystery, and the four men at Jerusalem. Acts 18:18; Acts 21:23 . These were probably the vows of Nazariteship, by the head being shaven. According to the law the final shaving must be at the tabernacle or temple. Numbers 6:18 .