Introduction
Vedavati is a woman ascetic whom Ravana insulted. She is said to have been reborn as Sita. Her story is told in the Uttara Kanda, and in the Yuddha Kanda Ravana refers to it.
This article primarily follows the Valmiki Ramayana. Where a well-known detail comes from a later retelling or tradition, the article identifies it.
The story
Vedavati was the daughter of the sage Kushadhwaja and wanted Vishnu as her husband. Ravana saw her practising austerities in the Himalayas and grabbed her by the hair. She said she could not live after this insult, and entered the fire, vowing to be born again as the daughter of a righteous man in order to cause Ravana's death.
Rebirth as Sita
In the Yuddha Kanda, Ravana himself recalls that Vedavati was reborn as Sita, the daughter of Janaka. Later traditions, including the Adbhuta Ramayana, develop the story further, and some add a "shadow Sita" motif.