The Czech, who also beat Vekic en route to victory at the Miami Open this year, earned her 31st career title.
The left-hander looked completely at ease on the surface as she captured her sixth title on grass, more than any other active WTA player, in a warning to her rivals ahead of Wimbledon.
It is the first time Kvitova has won multiple titles in a single season since 2019 and she has now won 12 of her last 13 matches on grass dating back to her title run in Eastbourne last season.
"I love grass, I love you," Kvitova told a thinning crowd at the Steffi Graf stadium.
"I have to say big congrats to Donna. You played amazing tennis. Not only in the final but the whole week. It was exhausting yesterday and today," she said.
Kvitova and Vekic had played twice on Saturday after a rain delay on Friday but it was the Czech who did not drop a set in the entire tournament.
She was too strong for the Croat in the first set, while Vekic struggled with her forehand.
Kvitova found herself a break down early in the second set but battled back from 5-3 and moved 6-5 up with an ace.
Vekic, who has shot up the rankings from 69th to 23rd this season following an Australian Open quarter-final spot and a title in Monterrey, forced a tiebreak.
She was beaten, however, with a thundering Kvitova forehand on the first match point.
