Vedvati was a daughter of a Brahmrishi named Kushadhwaja in Kritayuga (Satyuga). This Kusadhwaja was a son of Brihaspati.
Vedvati was very beautiful and her hand was sought by everyone in marriage but her father gave her hand to no one because only Lord Vishnu was acceptable to him as his son in law. Due to this a Daitya named Shambhu became enraged and he killed her father while he was asleep in night. Her mother also died due to depression and grief on his funeral pyre. Due to this, Vedvati accepted only Vishnu as her husband in her heart.
When once Ravana was wandering in a forest in Himalayan region, he saw a beautiful female ascetic with matted hair and wearing black antelope skin.
He enquired about her family and praised her beauty. She told her entire story and he told her to stop penance as she was beautiful, young and should enjoy life and remarked that penance was for old and ugly ladies. He proposed her to be his wife which she denied.
She rebuked him for his proposal and angry he caught her hair. She now totally enraged cut her hair with her hand which remarkably became like a sword.
She immediately lighted fire and said she no longer desired to live being insulted in such a manner. She warned him that she would return as his doom and will be responsible for his death.
She says she would appear as Ayonija (not born from a womb) and as daughter of a pious man. She then entered the fire.
Suddenly celestial flowers started raining from heaven and she appeared again on a lotus with fragrance of a lotus.
Ravana took her and showed her to the ministers, seeing her features, ministers said if she stayed in your house, she will be your death.
Ravana angry by this threw her into the sea. She reached the shore and she came when Janaka was doing ritual ploughing for his Yajna, out of mother Earth.
So, Vedvati was reborn as Sita with the sole purpose of destruction and death of Ravana.
Source- Canto XVII, Uttar Kand, Valmiki Ramayana