Vollmer had been the favourite coming into the final having set an Olympic record yesterday, a year after being crowned world champion in Shanghai.
Vollmer took 0.08 off Sarah Sjostrom's 2009 world record as she added the Olympic title to the gold she won in Athens in 2004 as a 16-year-old member of the triumphant 4 x 200m Freestyle squad.
Meanwhile, South African Cameron van der Burgh has won gold in the men's 100m Breaststroke in a world record time of 58.46.
The 24-year-old had qualified fastest for the final last night with the best time in the world in 2012 and an Olympic record.
Tonight he led from start to finish to take 0.12 off Brenton Rickard's 2009 record.
Christian Sprenger was second and Brendan Hansen third.