Vasari was 8 when Leonardo died, so everything he wrote about him was second hand, sometimes derived from brief earlier bios and some interviews with Leonardo’s associates. He is a fairly good guide to L’s artwork. For example, he describes the eyebrows on the Mona Lisa which later writers criticized him for, saying the portrait had no eyebrows. But high resolution photos of the painting from several years ago show that it did indeed have them, which either were painted over or faded over time. Vasari interviewed L’s companion, Francesco Melzi, who inherited a lot of L’s property after he died in 1519. From him he learned about L’s left-handed writing, a fact he added as a correction to the second edition of the Lives of the Artists in 1568. He also saw some of L’s notebooks then in Melzi’s possession.

He’s right that Leonardo was famous in his own day and he gets L’s chronology basically right—verified by references to L here and there in other documents, like contracts. Whether all the colorful anecdotes he tells about him are true (L released caged birds in the market, he made balloon animals, etc.) is hard to determine. For instance, he says that L died in the arms of Francis I but Francis was in Paris when L died in Amboise. Vasari thought Leonardo was practically more than human (though flawed by his tendency to leave things half-finished), so some of his stories seem crafted to highlight that point

He’s much more reliable about Michelangelo since he knew him well; he studied with him for a time and was his friend. The second edition of Lives is even more accurate because in the meantime M’s friend Ascanio Condivi had published in 1553 his own “official” bio of M., prompted by M’s complaints about errors in Vasari’s first edition of 1550. (Condivi’s bio had its own biases in favor of M.) Vasari incorporated Condivi’s revisions into his second edition. Many more of the anecdotes he tells about Michelangelo can be cross-checked as accurate.