Did it stop blowing through the center vent and start blowing through the defrost at the same time? If so, it's more likely you have lost the vacuum source which actuates the vents as opposed to any problem with the vacuum actuators themselves. Vacuum is used to both close the defrost vents and open the center vents via independent sets of actuators.

First step in diagnosing it is to pull the glovebox and expose the vacuum distribution block (on the lefthand side). Use a handheld vacuum pump w/guage (AKA Mityvac) to test each actuator - I think there are 7 lines. Also test the incoming vacuum source by hooking it up to the guage and running the engine; check to insure you have vacuum supply.

The individual lines from the vacuum distribution block are color coded; you can lookup the function of each using the search tool here - there are many similar threads.

Yes, you can block open the center vents by removing the louver assembly and using a prop rod or similar. But unless you also get the defrost vents closed, this is going to be of limited value.

Best of luck