my 201 Eurolamp is from Bosch.
the venthole is located on the removable back cover of the lamp. There is a hole on the inside of the back cover that finds its way into an external vertical rectangular shaft on the side of the back cover.
this means that it is *useless* to improve the seal of the back cover, using silicone or grease, if the objective is to avoid fogging. I believe that the seal is just for dust protection.
by the way, fogging can occur if it is very humid and moist air gets trapped inside, and then the temperature drops (e.g., the headlamp lens gets wet from the rain).