It's backstory is in Uttar Kand where it's written that a daughter of Sage Kushadhwaja was very beautiful and many men wanted her hand but he refused them all saying that he wanted Lord Vishnu to be her husband. A demon named Shambhu due to this killed Kushadhwaja and his wife due to grief entered fire. Unprotected Vedvati left for the woods and wore matted locks and black antelope’s skin and did penance to get Lord Vishnu as her husband.
Ravana once saw her and praised her beauty. She introduced herself and called her Vedas personified. He proposed her and called her penance unworthy as she was very beautiful and young. When she rejected his offer, he grabbed her hair which angered her. She cut her hair through her hand which magically became as sharp as blade due to her penance. She entered fire vowing to return and destroy Ravana.
She later was reborn on a lotus with fragrance of lotus. Ravana saw her and brought her in his palace but his ministers seeing her told him that she would be the reason of his destruction. Ravana got angry and threw the baby in the Sea which was taken to Mithila by the waves and when Janaka was doing ritual ploughing ,he found the baby (reborn Vedvati) in the line of ploughed field and therefore named her as Sita and adopted her as his child. He gave her to his Queen and reborn Vedvati became Sita, Princess of Mithila.
Ironically, if Ravana had not thrown the baby despite his ministers warning him, Sita would not be brought up by Janaka but Ravana. But, he was unworthy to become father of Mahalakshmi and therefore she chose Rajrishi Sheerdhwaj Janaka as her father and fate played it's course.
Source- Uttar Kand, Valmiki Ramayana