As long as you're putting nice organic matter out in nature for it to break down, be eaten, etc. I don't think you can really do it
wrong, though some methods are quicker than others.
Since you're concerned about the PVC, why not just plain
compost in place? I'm thinking dig the hole, fill it with kitchen waste, and bury it when done. NEXT! If a hole will collapse, just make a pit and turn it into a compost heap and shift after a period of time. As long as you have a place that consistently has proper moisture and food available, worms will stick around. They'll also promptly leave if the moisture or food aren't right.
And being that you're proposing something for outside, I wonder if concern about non-worm invertebrate participants is warranted, as long as they're eating the compost material (and not your veggies). Worms are friends, of course. But other bugs are there, they eat the compost material and
poop and die, too. I see tons of different creatures in the compost heap and in the garden during the summer. I take it as a sign of a healthy and varied little ecosystem. (It's really really extra awesome when a tree frog shows up to join in, too.)
I've been doing vermiculture for years now and use just big bins. I put a handful of worms and bedding from an established bin in the bottom of an empty, then add kitchen scraps and other stuff I want to
feed them along with high-carbon bedding (usually paper). When a bin gets full, I let it sit for a while for the worms to work through everything, then I dump the whole thing out on the garden. That jump starts the worm population, and as long as there is organic matter on top and it's not too cold or too dry, the place is seething with worms. It wasn't like that when we got here. My wife was concerned about smell and general disgustingness from the worm bins but she's come around because the results are so good.
