a/c blowing on windshield. help!

I have a '96 Hoe and the a/c blows partially thru the defrost vents which causes a real problem with moisture accumlulation on the windshield. It still blows thru the dash vents just not full forch as part of the flow is coming out the defrost vents. What can I do to fix this problem? Anyone have the same problem? Thanks

you have a vacuum leak in line with the actuator that controls the blender door