Vondrousova, a 2019 French Open finalist, advances with the 6-1, 6-4 win.
"How disappointed am I? I mean, I'm disappointed in every loss, but I feel like this one sucks more than the others," Osaka said.
Osaka -- who had 32 unforced errors in the match -- isn't the first big name to lose early in the women's draw. World No. 1 and Wimbledon champion Ashleigh Barty of Australia lost in the opening round to Spain's Sara Sorribes Tormo. No. 3 seed Aryna Sabalenka of Belarus also made an early exit, losing to Croatia's Donna Vekic in the second round.