Barthel, the defending Hobart champion and seeded ninth, defeated Kirsten Flipkens 6-4, 6-4 and Vesnina was an efficient 6-2, 6-2 victor over No. 8-seeded Sloane Stephens.
They square off Saturday in the tournament title match. Vesnina dropped just two games in winning her only previous match against Barthel last year in Beijing.
Vesnina will be making her seventh WTA finals appearance but is yet to win a tournament. Last year's 6-1, 6-2 win over Yanina Wickmayer in the Moorilla Hobart finals was Barthel's only other WTA title match.
Barthel broke Flipkens' serve five times in 10 games, which allowed her to overcome dropping her own serve three times.
Vesnina needed just 69 minutes to get past Stephens. She also had five breaks but lost her serve just once while winning more than two-thirds of the points on serve.
The Moorilla Hobart International champion will win $40,000, with the runner-up taking $20,000. Flipkens and Stephens each were given $10,425 as semifinalists.
Swim Week Miami Beach 2014 [PHOTOS]