The 33-year-old 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.
"I love grass, I love you," Kvitova told a thinning crowd at the Steffi Graf stadium.
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.
Persons:
Petra Kvitova, Croatia's Donna Vekic, Lisi Niesner BERLIN, Donna Vekic, Kvitova, Steffi Graf, Donna, Vekic, Karolos, Christian Radnedge, Hugh Lawson
Organizations:
Tennis Club, Wimbledon, Miami, Eastbourne, Croat, Thomson
Locations:
Berlin, Germany, Czech, Croatia, London, Wimbledon, Monterrey