A vow is a covenant made between two parties. God made a vow to mankind never to destroy the earth with floods and this is symbolised by the rainbow. Ecclessiastes 5:5 says it is better not to vow than to make a vow and not pay it. Judges 11:30 talks about Jephthah how he made a vow to God that in case God would grant Israel victory over the Ammonites, he would offer the first thing that would come out of his house on his return home. He returns and the first person to come out of the house was his only daughter. Rather sad, but he fulfilled his vow to God. A vow attracts opposition which God uses to prove our faithfulness to him. Similary to the wedding vows couples make,there will be difficult times in your marriage just for proof of your loyalty to God and your partner.