What does it mean that he fulfilled his vow?
Malbim citing Ta'anis 4a, Tanchuma (Bechukosai 5): He slaughtered [and offered] her [like an Olah].
Radak (31, 39): He made a house for her and entered her in it. She separated from people and from worldly matters. She was Kodesh to Hashem 1 - "Lo Yad'ah Ish" 2 , and she cried over her Besulim (verse 37). It does not say that he offered her for an Olah!
Ramban (Vayikra 27:29): You cannot say that it was a statute in Yisrael to cry over a girl who did not marry and served Hashem in Taharah!
Malbim: I.e. "Lo Yad'ah Ish" was the fulfillment of his vow.
What was the statute?
Rashi #1: It was a decree lest this happen again (Radak citing Targum Yonasan - that someone sacrifice his child). Had Yiftach gone to Pinchas, or Pinchas to Yiftach, this would not have happened. Refer to 12:7:1:1.
Rashi #2: It is like verse 40 says - girls would go (Radak - to her) to cry over her.
