First of all, congratulations on picking up your first T. If you're anything like many of us, it won't be your only one for long.
Now, on to your question.
Personally, I don't use vermiculite as a straight substrate. I will always mix mine with peat moss. For a G. rosea, it's probably not the best substrate due to it's high water retention abilities. G. rosea are a scrubland species. They do not need high humidity and in fact don't care for it very much. I would personally either use straight peat moss or potting soil for it. Your Rosehair will be fine with a dry substrate and a waterdish.