Original: He writes that when Cimabue was absent from the workshop, his young apprentice painted such a lifelike fly on the face of the painting that Cimabue was working on, that he tried several times to brush it off.
Simplified: Vasari writes that when Cimabue was away from the workshop, Giotto painted a fly on the face of the painting that his master was working on. When Cimabue came back, he tried several times to brush the fly off.