The campus Sceleratus was an open area, immediately inside the Servian all and south of the porta Collina, where Vestal Virgins who had broken their vow of chastity were buried alive.
...it looks as if the Temple of Vesta is not within the Piranesi perspective 'view' of the Roman Forum. The three columns with entablature belong to the Temple of Castor and Pollux, and the round Temple of Vesta would be right in the center of the view within the clearing in front of the two tiny figures. I'm guessing the temple base was still buried in Piranesi's time, and the few columns of the round temple that are in the Forum today are a reconstruction of 20th century vintage. Note, too, how the church facade seen through the three columns does not exist in our time.
The Roman Forum is not delineated within the Ichnographia Campus Martius, but Piranesi did draw a reconstructed plan of the Roman Forum within volume one of La Antichita Romane--that plan is nowhere close to what we know the plan to be today.
"The Vestals, priestesses of the goddess Vesta, whose temple was opposite their house, were selected among the daughters of patrician families between six and ten years of age.
They were exempt from the common law, had many privileges and great political importance, often interposing to save a life, or to restore harmony at times of crisis. They sat in seats of honour at public games and were almost the only citizens allowed to drive within the walls. Their duty was to watch by night and by day the sacred fire in the Temple, and to guard the Palladium. If the fire was allowed to go out, the Vestal in charge was scourged for her carelessness; if she violated her vow of chastity, she was condemned to be buried alive."
The place where Vestals were buried, the Campus Sceleratus, is delineated within the Ichnographia Campus Martius, but, unless you are looking at an actual engraving of the Ichnographia, you are not likely to discern its label:
I believe that I once read somewhere that it was a criminal offense to murder any virgin in Rome, thus the quandary of how execute a fallen Vestal, hence live burial at the Campus Sceleratus.