Can a clogged catalytic converter cause a rich idle & low vacuum?...

 

How clogged are we talkin'?
In my experience, a clogged cat affects high rpm or WOT situations, not idle with low exhaust flow.
Your low vacuum condition concerns me more.

 

I don't know the answer but I'll put this here to spawn discussion...

An O2 sensor measures an absence of oxygen. If the engine can't expel exhaust there will definitely be an absence of oxygen (reading rich) at the location of the O2 sensor.

If engine cannot expel exhaust then cylinder pressure will be greater when the intake valve opens and the MAP will report a numerically higher pressure (low vacuum).

You're thinking seems to be in the right direction....

 

A clogged cat can also glow red. Might want to give it the eyeball test when it gets dark.

 

I don't know that it's clogged at all, but it's 39 years old. Car is non-ECM, so no sensors or codes to diagnose anything.
'Been chasing my tail with this for the past year now. 'Originally thought it was the carb, but have tried 3 different carbs (2 Q-jets and 1 Edelbrock) and made no difference. Tried 2 different distributors, caps, & coils. No difference. Spark plugs are new. Ignition wires are new. Timing is set correctly. Have found no vacuum leaks. Exhaust stinks and burns eyes at idle. Still need to test fuel pressure, but swapped the mechanical pump (even though it was new) and still no difference. It's like a curse...