I agree that Fluttershy and Big Mac are way too similar for a dynamic relationship, but I let it slide because I like them both so much and because straight ships are too few to begin with.
Anyway, I would think that Octavia and Vinyl are actually too different. And besides, the funny thing is that in the show, they don't even live anywhere near one another. Not that I take ANY shipping pair very seriously. Aside from the canon married couples, the only ones that are even hinted at in the show are Cheerilee and Big Mac.
Still, I guess it amuses me to discuss it.