For those that are interested, here's a closer look at how to do this simple modification:
First, I took a Stormtrooper Prometheus figure apart, which is all easily done by simply starting with the chest piece & undoing the tiny screws in the back:
Then, I did the same with Vader's body which, for the most part, is just a larger version of the Stormtrooper body:
You can see here how much the abdominal pieces differ in size, which I measured to be about half-an-inch in height:
Here's Vader's body assembled with the smaller abdominal pieces:
Digging around the house, I found some nice thick black Gorilla Tape that could fill out Vader's otherwise emaciated waistline:
BTW, it is very important that the Stormtrooper's spring-loaded bolt with the square base is used to connect the pelvis to the abdomen. The bolt on Vader is designed for a smaller slot & it will slip out of the Stormtrooper abs, causing the figure to fall apart!
Once you slip his body suit on, it should still fit nice & snug even at this slightly shortened stature:
As Vader gets completely suited up, you can hardly tell he's even had any work done:
And, once the cape is on, voila -- a perfectly sized 1/6th-scale Vader! By my measurement, he's almost 14 inches, which I suppose works out to what a near seven-foot Vader should be:
Anyway, I don't remember seeing any similar suggestions here on how to easily resize this figure without the need for Dremels, messy glues or hack saws. Of course, any & all feedback to my solution here would be greatly appreciated! ;-)