interesting.
I should make it clear, I'm not saying using vermiculite as a substrate is going to kill/injure your G. rosea, its just a substrate that's normally used for humid loving species. That's its natural quality, the retention of moisutre. Infact Anne West (RIP) wrote a book on tarantula in the early 90s which advocated vermiculite as a good substrate, and she was one of the founders of BTS I believe. But, as I suggested, we've learned a lot more since then and the improvements suggest a substrate similar to their environment or at least one that provides the facility for the tarantula to use its natural desire to burrow.
If your tarantula is happy on vermiculite then I guess there's no reason to change, however its not something I'd suggest to a new keeper as the best substrate.