Roger,
I am heading towards 76 and it doesn’t get any easier….. I, too, am using Win10/VegasPro17(lastest build)/and the free stand-alone of pI, and I have the same problem as you seem to be having. I have closely followed this thread and have tried the things that were suggested but cannot get a particle with an alpha background. I have tried .mov and .mp4 as rendered formats and no matter what choice I select for the ‘Media Properties’ under ‘Alpha’, I still get a black background. What I have done, however, is add Chroma Key Fx and adjust the thresholds. It works – kinda. But have no worries that you are alone in this dilemma, there are at least two of us!
Bob