I discovered the same thing on my car--the vacuum tube coming off the ELZ AKR and going to nowhere at the back of the engine. It must have been like that for years. After I discovered it, I suspected it should have been connected to the intake manifold, somewhere, but I couldn't find any nipple that was uncovered, so I tapped into an existing vacuum line with a T-fitting. Honestly, it didn't seem to make a difference one way or the other.

__________________
1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles