John Gill Commentary Judges 11:36

"And she said unto him, My father, thou hast opened thy mouth unto Jehovah; do unto me according to that which hath proceeded out of thy mouth, forasmuch as Jehovah hath taken vengeance for thee on thine enemies, even on the children of Ammon." — Judges 11:36 (ASV)

And she said unto him, my father, [if] you have opened your
mouth unto the Lord
The conditional word "if" may be left out, as it is not in the original text; for her father had told her that he had opened his mouth, or made a vow to the Lord, and had no doubt explained it to her what it was, though it is not expressed; she knew it respected her, as it had issued, and was concerning her, as appears by her later request:

do to me according to that which has proceeded out of your mouth ;
which is a remarkable instance of filial subjection and obedience to a parent, and which perhaps was strengthened by a like mistaken notion as that of her father concerning the vow, that it could not be dispensed with; and therefore was moved under a sense of religion, as well as filial duty, to express herself in this manner, as well as by what follows:

forasmuch as the Lord has taken vengeance for thee of your enemies,
even of the children of Ammon :
such was her public spirit, and the grateful sense she had of the divine goodness, in giving victory over Israel's enemies, and delivering them from them, with vengeance on them, she cared not what was done to her; yea, desired that what was vowed might be performed.