I have a 91 300D 2.5 Turbo which is similiar to yours. Before I disabled that vacuum line 2 weeks ago, I had to touch the temperature setting dial so often in order to get the air out of the center vents. Whenever the temp dial's was set closer to the interior temp then the air will come out of there. As it cooled down then you have to reset it again and again and again....just to keep it flowing thru there.
May be you can try this. remove the glove compartment, you will see a vacuum distributor strip with all vacuum lines connected to it. locate and disconnect the 2nd vac. line(green) from top which has connection aligned horizontally. plug the supply end with a short vac hose and BB. this allow the air come out of the center vent at all time regardless what the dial setting is. I think this will let the Recirc flap open too because I don't feel the air is as cold as it was before. This weekend, I'll tape the Recirc air intake completely. I'll just roll a window down whenever I need some fresh air.
Hope this temp fix will help.