Leviticus 22:23

but for a vow The three varieties included under the name Peace-Offering were, as has been noticed (see on Leviticus 7:11), (a) Thank-Offerings (here mentioned as a separate item in Leviticus 22:29, (b) Votive-Offerings, and (c) Freewill-Offerings. The first (see W.P.Paterson, HDB, Art. Sacrifice, p. 338) was offered in acknowledgment of benefits received, the second and third were combined with prayer for blessings hoped for. These two differed thus, that the Freewill-Offering was simply in support of the prayer, and was made in anticipation of the benefit asked. The vow was promised on fulfilment of the prayer, and thus, unlike the Freewill-Offering, need not be offered if the prayer remained unfulfilled. According to this v., gratitude for the answer was to be indicated by the greater stringency of the regulation as to the nature of the animal to be offered in acknowledgment of the mercy vouchsafed.

For the general prohibition to offer a sacrifice that had a blemish, cp. the rebuke in Malachi 1:8; Malachi 1:13.