Vacuum Mystery ?

It's not uncommon for the engine to idle way down or even die when covering the PCV valve. It is in essence a controlled vacuum leak in normal operation. If the engine is tuned for the PCV, and I'm sure it is, if you disconnect it and plug it, it is entirely possible to get your result, since the resulting "plug" of your finger or thumb would stop the "controlled leak" completely and probably pull the idle down more than when the PCV is plugged in and working. It's also entirely possible it might cause the engine to stall. Sounds to me like the PCV system is operating normally.