It started misfiring on the back cylinder (closest to the firewall) and after checking spark and fuel noticed an open vacuum port on the intake manifold of the back cylinder. When I covered the hole the engine started running on all cylinders. I looked all over, with a mirror even, and saw no sign of a mate. I have the service CD and couldn't find anything other than a background indication.

Where does it go, what does it do (if anything)?

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5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)