The flesh (meat) of the cattle is the portion to the offerer (7:15-18). The flesh of the peace offering for thanksgiving is good for eating on the offering day (v. 15), whereas the flesh of the peace offering for a vow or a voluntary offering is good for eating for two days (vv. 16-18). This shows that the offering for a vow is stronger than the offering for thanksgiving. Thus, the offering for thanksgiving is good to eat for only one day. But the offering for a vow or a voluntary offering is stronger, so it can last for two days.
We need to see the difference between the peace offering for thanksgiving and the peace offering for a vow. The offering for a vow is stronger. Today we may consecrate ourselves to God with thanksgiving. We may pray, "Lord, I love You, so I consecrate myself to You." This is for thanksgiving, but this is too general. On the other hand, we may offer ourselves to God with a vow. We may pray, "Lord, I come here to make a vow to You. I give myself to You and marry myself to You. I want to be solely for You always, regardless of what happens or of how I feel." A vow is something voluntary.
A number of saints may consecrate themselves to Christ and the church, but five years later they may leave the church. This means that they did not have a vow. A vow is like a marriage tie. The offering for thanksgiving, though, is based upon our feeling. We may stay with someone because of our feeling of love for them, but a vow goes beyond our feeling. It is a tie that binds us regardless of feeling or circumstance. All of us need to be ones who are married to Christ for His recovery. Then regardless of what happens or of how we feel, we will always remain with the Lord for His recovery. I am grateful to the Lord that many saints are really bound to Christ by a marriage vow. To make such a vow is to be a real Nazarite according to what is revealed in Numbers 6.
The offering for thanksgiving is emotional and superficial, but the offering for a vow is determined and deeper. Thanksgiving is mostly related to the emotion, but a vow is related to the will. Some saints' consecrations are out of thanksgiving, whereas others' consecrations are out of a vow. In order to follow the Lord, we need a vow. Marriage involves a vow with no change, no variation, no alteration. The Lord treasures our vow to Him.