You will need to locate where the vibration comes from. If it's the mount, you can use thin strips of dense foam or rubber (strips of bicycle inner tube work well) to help isolate the cameras. If it's internal in the cameras... Not sure as of yet. How do the cameras mount to the bike?

Since you said it's not an issue when the cameras are on your helmet, I'm willing to say vibration is causing the issue as your body is absorbing the vibrations.