‘ And so it was that, when he saw her, he tore his clothes and said, “Alas my daughter, you have brought me very low, and you are as one of those who trouble me, for I have opened my mouth to Yahweh and I cannot go back.” '
When he saw who first came from his house he was devastated. He ‘tore his clothes', an expression of great emotion and deep grief (compare Genesis 37:34). He was not blaming his daughter. He was simply letting her know how deeply he felt the consequences of his vow. But his firm faith comes out in his final words. He intended to fulfil his vow whatever it cost him.
The question of vows is a complicated one. Numbers 18:14 says, ‘everything devoted in Israel shall be yours (that is, Aaron's).' But it makes provision for the fact that a human being who is ‘devoted' (strictly set apart as Yahweh's) can be redeemed (Judges 11:15). On the other hand Leviticus 27:28-29 says that anything ‘devoted' must be put to death without redemption. The distinction lies in the meaning of devoted. The latter has in mind when Yahweh has devoted something to destruction (Joshua 6:17 following; Deuteronomy 20:16-17; Numbers 21:2-3 - Hormah means ‘devoted'; 1 Samuel 15:3 onwards). In that case there is no remission. The former means something ‘devoted' because of legal requirements such as a firstborn, or an oath, when the provisions of the law must be followed whatever they be.
But in Leviticus 27:1-8 provision is made for a rash vow to be redeemed. The price of redemption for a woman would be thirty shekels. On the other hand Numbers 30:2 declares, ‘When a man vows a vow to Yahweh, or swears an oath to bind his soul with a bond, he shall not break his word. He shall do according to all that proceeds from his mouth'. Once put into words it is binding (Deuteronomy 23:21-23). Much clearly therefore depended on what type of vow was in mind. The latter would seem to have especially in mind a vow like Jephthah's, one made solemnly to Yahweh. From that there was no escape unless it was contrary to Yahweh. However, in the case of a vow to do something displeasing to Yahweh - a vow could not be paid with ‘dirty' money, nor, we must assume, with something that was an abomination to Yahweh (see Deuteronomy 23:18) - it is probable that Leviticus 27:1-8 would be applied.
This would suggest that while Jephthah's vow was heartrending, it was pleasing to Yahweh, otherwise Leviticus 27:1-8 could have been invoked. And it thus points to his daughter becoming ‘sanctified to Yahweh' in the Tabernacle, ‘offered up as a whole offering' in the offering up of a lamb, and then becoming one of those of whom Yahweh would say, “She is mine”. This rather than actually being sacrificed in a way that could not be pleasing to Him, indeed was an abomination to Him. Jephthah's ignorance or otherwise does not come into this. He would certainly not have been short on advice and guidance about the matter.