Because of Age + mileage, the plastic, rubber, and seals will go bad in time.
Most of the vacuum leaks are usually caused by a bad EGR, or PCV system.
A good smoke test can pin point your vacuum leaks.
See Google ''Youtube Auto smoke test for vacuum leaks''
By isolating the PCV + EGR system, you can pick up some lost horse power.
''In the car valve stem seal replacement'', it isn't worth the time, because of the high mileage.