Terms of Service--basically, what you're agreeing to by using the software. Though I'm pretty sure that barring VX resources would've been precluded in the agreement since said resources didn't exist at that point, unless VX has some kind of retroactive clause seeing as how you'd technically need to own VX (and have agreed to its ToS as well) to legally use those tilesets. I ain't gonna backseat mod you or extoll some legal virtues, but suffice to say, that's terribly bad form if you don't also own VX.
It's not even that RTP looks too medieval--people have made non-medieval tilesets if you're willing to look your ass off--it's that VX just looks cartoony, for lack of a better word, and has eye-destroying wall autotiles. But if that's the flavor you're looking for, then that's the flavor you're looking for. And I don't know about "easier," since from what I understand, VX has its own mapping autofeatures that won't work in XP because it runs a different set of programming, so using a VX tileset in XP will be just as hard as using any other tileset in XP. It might even be harder, since the visual elements in VX maps are designed for a different system entirely.