From Robert Alter’s commentary:
The narrator, like father and daughter in the dialogue, avoids spelling out the terrible act of child sacrifice. This whole story has parallels elsewhere in the ancient Mediterranean world, the most obvious being Agamemnon’s sacrifice of his daughter Iphigenia in order to obtain favorable winds to sail to the Trojan war. The parallel episode within the Bible is the Binding of Isaac, but here, in contrast to Genesis 22, the ending is tragic.
From Francesca Stavrakopoulou’s “King Manasseh and Child Sacrifice”, from a passage comparing it to the Binding of Isaac:
It is significant that the label יחיד is also applied to Jephthah’s daughter, who is sacrificed by her father to YHWH in fulfillment of a vow. The narrator is keen to emphasize her high status as Jephthah’s only begotten child: “She was his only-begotten child (יחידה): apart from her he had no son or daughter”. Another feature common to both stories is that the only-begotten is to be offered up to YHWH as a עולה, a burnt offering, just as Mesha offered up his firstborn as a burnt offering. However, whereas a ram is sacrificed in place of Isaac, no such substitution occurs for Jephthah’s daughter, for Jephthah “did with her according to the vow he had made.”

In light of the earlier discussion concerning the biblical language of child sacrifice, it is particularly striking that the verb עבר, associated with both the firstborn sacrifice and the so-called “Molek” ritual, is employed three times to describe Jephthah’s movements immediately before he makes his vow. Moreover, the text implies that Jephthah’s actions may be prompted by the descent of the spirit of YHWH (רוח יהוה) upon him.
Given that both Abraham and Jephthah are consistently depicted as heroic figures faithful to YHWH, it is highly significant that in neither story is the practice of child sacrifice condemned, nor even remarked upon by the narrator.
The Iphigenia tradition is a solid parallel to the Jephthah story. Taking the story on its own terms, it is hard to avoid concluding that Jephthah’s daughter is sacrificed and the relationship with the Binding of Isaac suggests that the latter serves as a counterpoint to Jephthah’s situation. In the case where the sacrifice is not carried out, the demand came from God, whereas in Jephthah’s case, he alone made a demand that was binding on himself. I think the story is pretty deft because Jephthah returns victorious and yet it is his daughter who is commemorated yearly, while Jephthah, the actual war hero, is remembered as a fool who made a rash vow. That Molek sacrifices were so despised by the Deuteronomists makes this didactic lesson even more of a gut punch.