Mattek, a 23-year-old from Miami, next plays teenager Yanina Wickmayer, who defeated Briton Melanie South 6-4, 6-3 in their quarterfinal match. The only other semifinal for Mattek was at Cincinnati in 2005, and she has not won a WTA tournament.
The other semifinal will be between 12th-seeded Kateryna Bondarenko and Marina Erakovic, who defeated Bondarenko's younger sister, Alona, in the quarterfinals.
Vaidisova, ranked 19th in the world, failed to convert two break chances while trailing 5-3 in the first set. She lost that game and then the 81st-ranked Mattek swept the second set.
Wickmayer, an 18-year-old Belgian, will be playing in her first WTA Tour semifinal. She had to face the pressure Friday of a crowd that favored her opponent.
"I was playing in front of a crowd with not many people on my side," Wickmayer said. "I was a little lucky when my opponent missed on one or two important points in the first set, but I hit the ball much better in the second set and I fought very hard."