The OE cap has a spring that will allow the tank to hold some vaccum. The cap won't vent to the out side air but will let air in. Some have posted that they have had a psi build up in there tank and have drilled a hole.
When you remove your cap do you notice a small amount of vaccum? My system has always had a vaccum.
I'd say a non-vent but it must be able to allow air to inter tank.