If you remember, these vellums used to be made with different types of inks, based on how high level the enchant you wanted to put on them was. There were also weapon and armor vellums. Now, the system has been simplified so that a single vellum can accept any enchant; it is made from Moonglow Ink, which is one of the inks you can make by milling the incredibly cheap starter zone herbs.
Blizzard typically doesn't take something that's craftable and move it to an NPC vendor; however, it sometimes does this to ensure availability of the product, as well as introduce another outlet (or sink) for gold. We have seen a few exceptions to this trend of late, and I suspect it's because the game designers are valuing the general availability of products like Dust of Disappearance and these vellum more than they value the player-driven market of these items. If it makes more of their customers enjoy the game more, then it's a good design decision.
That said, Blizzard typically prices these items in such a way that it's still possible for players to make them for a profit. People are used to paying gold for vellum; however, that price was based on needing Ink of the Sea for them. I know that on my realm, prices didn't go down that drastically after the change, but assuming an average value of 3g for stack of low-level herbs, your Moonglow Inks are going to cost 60 silver each. This means that there's no room for players in this market, except for buying the vellum off the vendor and listing it on the AH.