No, you should not use Swiffer WetJet solution, vinegar, or any other all-purpose cleaners in your Braava jet m6. Using other solutions can damage the robot's internal pump and tank, and might void the warranty.
If you have run out of the iRobot cleaning solution, you can use warm water by itself. Fill the tank completely with just water. For a normal cleaning mix when you have the solution, use 2 fluid ounces of the iRobot solution first, then fill the rest of the tank with water.
Also, do not pre-wet the cleaning pads before use. You should use a new, dry pad for each cleaning job.