Cause and solution to collapsed vacuum line to "Crankcase Ventilation Primary"

I'm going with the idea from comments that the cause is simply the age of the hose in question. I also found the the plastic assembly where the four hoses connect was full of grit and grime. I cleaned it with carb cleaner.

The hose in question routes just behind the passenger side valve cover. Follow the secondary vent tubes out of the back of the manifold. They meet in a "Y" and continue into the valve.

Just to the right of that, there's a brass looking fitting over which the primary vent tube fits. See the image below.

To get the right size hose, I suggest taking the plastic assembly off the top of the manifold and taking it to the parts store to ensure a perfect fit. You'll need somewhere around 20 inches. My parts store sold it by the foot. I got 2 feet and didn't have to cut it down.

While still imperfect, replacing this hose and doing some throttle body cleaning while I had it apart made a big improvement in the way the engine runs and idles.

Other angles available here