Reverse order -
AC issue is due to loss or lack of vacuum.
Getting a vacuum diagram to trace the lines and parts is the best next step.
Likely there is a vacuum reservoir in the system with a check valve. These may be combined or separate.
You will need a vacuum pump or gauge to test.
It is common that the check valve is not sealing.
Other areas would be at the end of a connection. Cracked or split ends are common as well. Treat those and all rubber/plastic pieces with a conditioner.
Yes play in any drive train piece can cause a vibration
What speed? If you accelerate past the speed and then drift down to that speed in neutral can the vibration still be felt?
Action
AC issue is due to loss or lack of vacuum.
Getting a vacuum diagram to trace the lines and parts is the best next step.
Likely there is a vacuum reservoir in the system with a check valve. These may be combined or separate.
You will need a vacuum pump or gauge to test.
It is common that the check valve is not sealing.
Other areas would be at the end of a connection. Cracked or split ends are common as well. Treat those and all rubber/plastic pieces with a conditioner.
Yes play in any drive train piece can cause a vibration
What speed? If you accelerate past the speed and then drift down to that speed in neutral can the vibration still be felt?
Action