Flashlight occupy 5% on the vest. Yet it is attached on it and should not occupy any storage space.

Example :
Take a RU grenadier vest. Fill it with 45rd RPK74 clip, you can fill 10 inside and attach the flashlight to the vest.
Now empty it. Attach the flashlight, 5% storage space used. You can fill 9 clip on the vest.

When you look at the interaction, you'll see the flashlight push the vest storage space used to 105% but it will accept it and make it use 0%.
So either it should use storage space, either it should not. Whether you attach it before or after filling the vest should not impact the storage capacity of the vest.