According to Livy, Plutarch, and Aulus Gellius, the second king of Rome, Numa, appointed the first Vestals and laid out their responsibilities.* New Vestals were taken when they were 6-10 years old and served for 30 years. After this they retired and could marry if they so wished. It was considered good luck to marry a former Vestal, but Plutarch also says that many of them refused to do so and were happier for it.
The Vestals were mainly charged with caring for Vesta’s sacred fire, guarding sacred objects and the wills of important people, and preparing the mola salsa, a special flour sprinkled on all public sacrifices. They were watched over by the pontifex maximus and the chief Vestal, who had a seat in the College of Pontiffs and many or may not have been subject to the 30-year limit on a Vestal’s service.
As anyone who talks about the Vestals will mention, the punishment for breaking her vow of virginity was to be buried alive. It is worth noting though that all ten recorded accusations of unchastity took place during times of upheaval in Rome and it’s likely that all of them were politically motivated. Nor did all of them end in the death of the accused. Several were acquitted.
These women held a number of privileges. They could act without a guardian and make wills. Seats of honor were reserved for them at all public games and performances. Their carriages had right-of-way in the streets and a person sentenced to death who accidentally encountered a Vestal was automatically pardoned. The evidence given by a Vestal was trusted without her needing to take an oath. By taking on the priesthood of Vesta they became more than ordinary women, holding extraordinary status and responsibility.
*Livy’s claim that Rhea Silvia, mother of the founders of Rome, was a Vestal herself would seem to contradict this. Given the fact that he, Plutarch, and Aulus Gellius were all writing 600-700 years after the events they describe, we can pretty safely assume that the details of these accounts are more legend than fact.
Plutarch, "Life of Numa Pompilius" 9.5-10, 2nd century CE - Diotima
Aulus Gellius, Attic Nights 1.12. L, 2nd century CE - Diotima
Livy, The History of Rome 1.20, 1st century BCE - Perseus
The Laws of the Kings, 7th century BCE - Diotima
The Twelve Tables (excerpts). Rome, 450 BCE (traditional date) - Diotima
On Guardianship (Gaius, Institutes 1. 144-5, 190-1) - Diotima
An Inscription in honor of Coelia Concordia, Chief Vestal - Diotima
Vestal Virgin - Wikipedia
Vesta - Wikipedia
Oldest Roman Hairstyle Recreated for First Time - Live Science
Vestal Hairdressing - YouTube