Tenth seed Errani, 25, was playing in her third Grand Slam quarter-final of the year while best friend Vinci, four years older, was making her last-eight debut at a major.
The pair have won seven doubles titles in 2012, a haul which includes the French Open, but it was Errani's greater big-match experience that proved decisive.
"It's always hard to play against a friend and it was strange seeing her on the other side of the net," said Errani, who has four singles titles to her name this year.
"I am happy to get to the semi-finals.
Can I win the title? I will tell you in three or four days."