How did Rhea Silvia become pregnant if she was a vestal virgin? Did she have sex to conceive the twins?

Most historians, when studying the myth of Rhea Silvia, use as source the writings of the famous Roman historian, Titus Livius, more popularly known as Livy. He had written a monumental work on the history of Rome, and covered the period from its earliest legends before the traditional foundation in 753 BC through to the reign of Augustus when he (Livy) was still alive. Scholars have respected his version of early Roman history as being the most accurate.

Rhea Silvia was the mother of Romulus and Remus. The circumstances of their conception have been much debated, but Livy states in his account that Rhea was raped. She had been forced to become a vestal virgin and undertake a vow of celibacy for thirty years. This was done by her uncle Amulius, who seized the throne from her father, Numitor, and killed her brother to prevent him from being rightfully challenged by him or his heirs. Amulius was imprisoned.

In terms of the myth, Rhea Silvia fell pregnant after being violated by Mars and gave birth to the twins who would eventually establish Rome. Livy states, however, that she had been violated by a man, not by Mars. He assumed that her claim to have been raped by Mars was either a result of her imagination or because it was deemed less shameful for a vestal virgin to have committed such an offense with a god. Be that as it may, the punishment for such an act was death, and Amulius ordered that she be buried alive for breaking her vow of chastity.

In a number of myths, the river god, Tiber, took pity on Rhea Silvia and rescued her from Amulius's clutches. He later made her his wife, which promoted her status to that of a minor deity. 

Amulius had also instructed that Rhea's illegitimate twins, Romulus and Remus, be executed by exposure, but the servant who was to execute the sentence took pity on the two boys and set them adrift in a basket down the river Tiber. Their basket later got caught on the river bank, and they were rescued and weaned by a wolf. A shepherd and his wife later discovered them and raised them. As grown men they avenged their uncle's murder by killing Amulius and returning their grandfather, Numitor, to his rightful place on the throne.