Steve,
I have a 'cheap' HF 3.5 cu foot concrete mixer. It works fine for larger batches.
I do find that if I mix dry ingredients about 15 minutes before I add any water the vermiculite is well mixed with the port and sand. (If I add peat, I add that in 19 minutes and and mix about 5 minutes more, then add my water). I also have an old car ice scraper, that I use here and there to scrape the sides...but it works wonderfully! I mix a total of a half an hour. 15 min dry and 15 wet.
I've had no problems. But I only mix my trough batches and larger batches in it. I mix to a consistancy of a handful squeezed, holds it's shape and just a bit of water comes out. I have added a bit more port, thinking it wouldn't set, but it has been fine without the extra.
Otherwise I do by hand.
(In this heat...a mixer is the way to go!
Marcia

I can't help with the mixer question, but I am wondering about what is in your mix. Have you used only vermiculite and cement in the past so you know this mix is just what you want? Are you really are mixing five cubic yards of mix?
I always measure out my ingredients by volume using one part cement to at least three of aggregates. Perhaps one bag of cement and one bag of vermiculite is OK. This is just a word of caution

So far I've only tried a few small things. Still on my first bag of Portland. The items made with vermiculite in the mix have been the best looking in terms of looking rock like. Are there any peculiarities of vermiculite I need to be aware of?

Still got a lot of perlite to use up, but plan to order a large bag of vermiculite when I get to that point. I've made a bunch of "rocks" using plastic quart milk jug bottoms cut off about 4" high just to test for strength and color of different combinations. Glad I did it. Sure have been useful in other ways to hold down newspaper mulch, to prop up this and that.

achieved only 1 n/ mm2 for vermiclite light weight concrete is ok for light weight concrete?