Intermittent operation problem solved.
The engine finally "did me a favor" by stalling in a handy spot and then refusing to restart.
There was fuel at the filter after cranking, Checked for (cranking) spark at the plugs and there was none. Checked for spark at the coil wire and there WAS spark. That quickly narrowed it down to the distributor, which as lottsa Vigor owners know isn't too easy to access because it sits very close to the firewall.

As I'd mentioned in my initial posting of this situation, I wanted to check for oil inside the distributor cap to see if I did a sloppy job of replacing the O-rings when I did it some 15,000 miles ago. I saw no external seepage/weepage in this area and there was no burning oil smell as there was the first time I had to change the O-rings.

Instead of yanking the battery and the heat shielding to get at the cap and then the distributor, I made a crude but handy oil dipstick from a white plastic tie-wrap. Vigor distributor caps are vented through a pair of rubber tubes to the space underneath the fuel rail cover. (I've heard that venting the ozone created by rotor-to-cap terminal arcing makes for a friendlier atmosphere inside the cap for the current to flow.) Using my old spare cap as a guide, I pulled the rubber vent tubes off and rough-positioned the old rotor inside the (inverted on the bench) cap and stuck the tie wrap into the right/passenger-side vent hole. Because of the flexible wrap's natural curve (toward the serrations), I was easily able to push it in through the vent hole. A li'l low-tech jiggling and the tie-wrap practically followed the cap's contours around it's bottom.
Then I took a fresh, pliable white tie-wrap, extra long at 11 1/2-inches, gently pushed it down into the same vent hole on the cap on the engine. Withdrawing the tie-wrap after little more than the tip slipped in clearly showed that it had indeed struck oil, that my previous repair work was poor, and most importantly, it solved my intermittent ops mystery.

Pulled the cap and found oil in the cap and on the rotor. The cap's center carbon electrode post was almost worn down completely. It wasn't stuck in; it was wore almost flat with the inside surface of the cap! Replaced the O-rings with new ones that had a 0.012-in. too-small material diameter, but the good Buna-N rubber and I'd assumed okay temporarily. Re-installed an old cap and rotor (the OE pieces originally installed in Japan in late 1992 and now with w/75K miles), and the car fired right up and ran fine for the 500 miles it was needed for before a dealer could get me a new cap, rotor and replacement O-rings. A fillup along the way (after rural driving in very hot conditions w/some a.c. use) returned the Vigor's typical 25-plus mpg.

Thanks to any and all forum members that took time to at least look at some of this huge post and gave it a moment's thought. And thanks too, to member/moderator AcuraTL, for reading everything that I posted and for responding quickly with input.
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