I had heard this version of Ramayana but had shooed the thought away because the Ramayan soaps/serials that were aired didn't have this story. My grandmother too, for the fact didn't tell me this. I think someone is my school/ my mom told me this story.
One more thing, I like Lord Raam very much but have always wondered, why on earth did he suspect Sita and doubt her? Isn't this proving him to be like any other man? This story shows that too happened for a reason. Here it goes...
Ravan's eyes fall on this beautiful lady Vedavathi, who is an incarnation of Goddess Lakshmi. He tries to molest her and own her. She struggles to escape from his clutches and is finally saved by Lord Agni. She curses Ravan, & vows that she will be the reason for his death. Agni helps and protects her in his home under the care of his wife Swaha.
When Ravan again desires another woman who is now Sita and kidnaps her, Sita tries to escape from his clutches and seeks help from all Gods. Again Agni comes to her rescue. He replaces Sita with Vedavathi. [Both being incarnations of Lakshmi, its said they have very similar appearance as well and hence Ravan lusts for Sita too.]
It was Vedavathi who went through all the torture Ravan bestowed her in Lanka while Sita was with Swaha devi. Finally after the war, after the death of the demon, Lord Raam knows that this cannot be his wife Sita and summons for Agnipariksha to test her purity. Here she will be set ablaze and if she returns alive, then she is a true Pativratha, else she's not. Sita agrees for this test where Lord Agni takes in Vedavathi and returns real Sita sound and safe.
Thus proving Sita's loyalty/purity/devotion to husband and also fulfilling Vedavathi's desire leading to "Loka Kalyanam".