I'm not sure about the side vent but your loss of the center vent with accompanying air from defroster is a common problem. The system defaults by spring pressure into the defrost position, for safety reasons.
The vacuum servos that close the defrost vents and open the center are all on the same vacuum source. Thus any loss of vacuum including ruptured servos (2 defrost, one center vent)or open lines or mulfunctioning switch over valves or bad source vacuum will cause this problem. Usually it is one of the servos.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician