While I don’t have the ‘68 Ghidorah, if it’s anything like the ‘64 Ghidorah (which I do have), the wings don’t feel like vinyl. They are much harder and feel more like styrene plastic or something similar. Which would make sense because there’s no way you could make such large and thin wings out of the softer vinyl used for the body without MAJOR drooping issues. I suppose that they could be made out of harder vinyl akin to vinyl records (which can also break...). But my gut tells me they are plastic.
But whatever they are, they are very large, heavy, and quite thin. The material feels decently brittle. So, while they are sturdy enough, it’s not a vinyl bandai toy that can be handle roughly... if you drop this guy or bump him too hard in the wrong way, the wings are totally going to break.
Plus, vinyl can totally break. Vinyl can be made in a very wide range of hardness or softness. The harder vinyls (like records, or even the old 80’s-early 90’s bandai figures) can be broken quite easily, especially if dropped on a hard surface.