There is only one reason why the unit is starting to fog up when being used and that is moisture getting inside. How it got inside could be either a combination of things or one of the reasons below:

Lack of maintenance/service of a unit

Improper use/storage

The very first thing to do when fogging occurs is to service all accessible O-rings that your unit has. Remove them from the unit and clean the area where O-ring sits, after that clean O-ring itself with fresh water without any soap or other compounds. Apply silicone grease to O-ring(s) and place it back to original position.
Some units have not only main O-ring, but also a port O-ring (if the port is interchangeable), so make sure to look at that too.
If all done correctly, fogging should be resolved.

The second thing to prevent fogging is to avoid opening a unit outdoors. Every time the unit is opened (especially near water) the moisture is getting inside, due to high moisture concentration in the air. In fact, the best thing to do would be to prepare the unit at home (cool & dry place with low humidity), close the unit and not open it until your shooting day is complete. If for some reason you need to open the unit, please find the nearest air-conditioned place where it is dry and cool (a car with running AC will do the job). Another important thing is to never open the unit on the beach with wet hands and try to take out the camera.

To summarize the list of things to prevent fogging please see below.

• Avoid direct sunlight for prolong time without cooling
• Avoid opening the unit outside
• Avoid shooting 4K videos for too long
• Avoid touching the inside of a unit with wet hands

Units that are compatible with VPS-100 system will benefit from this accessory, because this accessory takes out all air from the unit and with it all moisture is gone as well, making it a secure vacuum seal.