why would this be a misuse? please elaborate on this. if all you are looking for is mass than this should do the trick (albeit it's not as dense as vinyl).
i'm trying to find out if there is something intrinsic in the vinyl *material* (desregarding density) that is better than butyl for barrier duty.
Would you use a screw driver to butter toast? A butter knife to drive screws? You could, but it's not what they're designed to do. The reason I call it as misuse is simply because it will not be doing what it was designed to do.
I understand your thinking tho, it's hard when just reading about the stuff on the web. Once you get it in your hands and work with it things start to make more sense.
Keep in mind that MLV isn't just any ol' vinyl. There's three letters in that acronym for a reason! It's much thicker and heavier than anything you've seen on a camper top. When cutting it with a carpet knife, the blade dulls quickly and develops a notch where it passes through the stuff.
But putting your idea to the numbers, here's some comparisons:
MLV = 1.2lb/sqft plus a closed cell foam decoupling layer attached
foil backed butyl:
40mil = .35lb/sqft
60mil = .45lb/sqft
80mil = .60lb/sqft
So you'd need to cut and fit a decoupling layer of some sort, either closed or open cell foam. Then cut and fit either 3-4 layers of 40mil, 2-3 layers of 60mil, or 2 layers of 80mil. That's a LOT of work and you still wouldn't have as good of results. I wouldn't go that route even if someone gave me the 80mil free.