WIll the center upper vents work if you use heat?.....the problem is not relevant of the A/C....it is just the fact that you notice it when using a/c because the air is directed upwards.
Based on the Transport diagrams, IN as much as vent control is done, the air is sent upwards as a whole to all panel outlets so if it goes to the side outlets....center should go also if the exit louvers themselves are opened, unless a separate duct is disconnected behind the scene...I do not have a clear picture of the ducts themselves. I do not think the silhouette variant to be much different. But just in case, the vacuum control relays are located on passenger side, to the left under the glove box.
bottom-left below
www.avigex.ca/xport/convenience_photo.jpg