You can rent a bike around the entire lake and here are some of them:
Velostore, Karlstad, + 46 76 191 98 41
Team Sportia, Kristinehamn, + 46 550 107 01
Sjötorps upplevelsecenter, Sjötorp, + 46 501 511 33
Evalds, Lidköping, + 46 510 204 80
There are also organizers of bicycle trips around Lake Vänern.
Global Runners offers an 8-day Lake Vänern package. (the package does not follow the route of the Vänerleden, entirely)