Dane,
Please don't blame yourself. I'm sure that what happened wasn't from something you did, or didn't do..... it happened just too quickly for you to have done anything about it! I truly think that he may have choked, or aspirated vomit into his lungs, and if so, that is not anything that you had any control over, and even if you had realized that it was happening, I doubt that there was anything that you could have done to help him.
Also, I doubt if a wasp was the culprit either, as my oldest male, Ming, ate one outside last year, and it didn't phase him at all, although I'll never know why the thing didn't sting him on the way down. And there is always the possibility, as Tracie said, that he could have had a heart attack or a stroke from an underlying condition that his own vet didn't even know he had. Bearded Dragons are very adept at hiding the fact they don't feel well, and even at hiding the symptoms of very serious illnesses, and, because of that, many beardies have died when their owners never even realized that they were sick. Unfortunately, that's just the nature of the beast.....
Try to find comfort in the fact that you took good care of him, and gave him lots of attention and love while he was in your care......so many of them NEVER have that! He was one of the lucky ones, with an owner who loved him, and it was the quality of his life, not the quantity that really counted..... I know you need time to grieve for him, but when you're feeling better, I do hope you'll consider getting another one. I truly believe that the best tribute that you can pay to a pet who has passed away is to give another of their species a good home. Truly good homes, with owners like you, are hard to come by for Bearded Dragons....a good home would be a terrible thing to waste, when you have so much love to give another....
at least think about it......and, again, I'm so sorry for your loss.
Denise