Well, there are other ways to get vitamin D3 (such as exposure to sunlight, or artificial
UVB light sources; however, most UVB bulbs, other than mercury vapor bulbs, do not provide enough of a concentrated stream of UVB to allow bearded dragons to produce the necessary amounts of vitamin D3, though they do provide some), but other than that, I had this question a while back as well, as I feed Reptiworms as my staple. From what I was told, and from what I understand, Reptiworms have a proper ratio of calcium to phosphorous, coming in at 1.43:1, where the typically recommended ratio is anywhere from 1:1 to 2:1. So when Reptiworms claims that they do not
need to be dusted, it is true, because it already falls within that range; however, I wouldn't say that that necessarily means you
cannot or should not dust them, as 1.43:1 is still a ways away from 2:1 (note, however, that "a ways away" is not indicated to imply that 1.43:1 is significantly far from 2:1; I'm just saying that there
is a difference).
However, I am not saying that you should dust your Reptiworms, as, unless tested, there isn't really an efficient way to determine whether or not dusting them still falls within the 2:1 ratio, or if it exceeds it. Too much calcium can cause problems; calcium-wise, you're probably safer not dusting your Reptiworms with any calcium supplement then dusting them. Unfortunately, however, this doesn't quite help with the D3 issue, but I'm just saying that it could be possible that you could safely dust Reptiworms without going overboard on the calcium while still providing D3.
I was also recommended to continue dusting his vegetables with calcium with D3, as he eats less vegetables than protein at this age, and therefore, the likelihood of over-calcification is lower than if I dusted all of his protein. This still provides the calcium needed to increase the ratio of the vegetables, as well as provides some source of D3. Other than that, however, I have inquired about and searched for pure vitamin D3 supplements, with little results, unfortunately.
Other suggestions have been to provide variety of feeders, including the ones that still need to be dusted with calcium and D3. Additionally, a lot of protein sources and living creatures have vitamin D3 created within themselves based on light exposure and diet, so I think what techdave was touching upon was that there could be the possibility that Reptiworms could also have adequate amounts of D3 already within them.
All in all, dusting is oftentimes very difficult to gauge, as it is difficult to tell whether one food has enough or not enough, or whether or not what you've added is too much or too little. I think the important thing is to gauge the ratios, and make sure that your bearded dragons have the proper diet and lighting in order to process (or properly pass, in the case of over-supplementation) their vitamins and minerals.