yeah, there's no such thing as "outside world exerts force on human, human does not exert force back". newton's third law and all that.
touch is a good example. me touching something means that a force is exerted on my skin, but likewise i'm exerting a force on that object, too.
as you did, you might be able to say that the vestibular system is just a very refined touch sensor. in that case, i might suggest that internally they're wired up very differently in the brain, to sense different things, although they both boil down to forces.
but, we can technically boil all(?) of our senses down to electromagnetic interactions (photons exciting rods/cones, electrons repelling each other - touch, chemical reactions - smell). so if we want to do the reduction game, we should say we only have one sense... the sense of electromagnetism!
at any rate, since you need different parts for an accelerometer and a force gauge for electronic purposes, we should probably give the sense of balance the benefit of the doubt! i dunno, maybe we should vote about it.