Just thought I'd close out this thread with the results. I got a new (aluminum) evaporator (the original was copper), had it installed, and the system swept, vacuumed, and recharged. Everything is working great. The compressor is finally cycling on/off after a fairly long initial cooling cycle. No more vacuum condenser light.
I have a couple more questions if anyone is inclined to reply--just for my own edification.
On this 642 the tech used between 8 to 8.5 oz of R134a. The history of the unit is unknown due to a lack of s/n, so the tech had to estimate the proper charge amount based off a standard table. How sensitive is the refrigerator to getting the optimal amount of refrigerant? Is it really to the ounce? Does an overcharge cause the compressor to work harder, the current draw on the compressor was ~1.3A, and the tech thought he'd bleed out a little refrigerant because he wanted to see it closer to 1A.
Last question, is there a typical cooling/defrost cycle on the S/Z 642, or is it all based on sensors? How long should I expect each cycle to be, or can that question even be answered?
Thank you all for your input. It was extremelly helpful. Now how do I get a beer to you?