Walking on a VR treadmill is actually the opposite example of what you are claiming I believe. It exercises your proprioception without giving you the right vestibular response (except on huge military 12x12ft treadmills and stuff that can accommodate decent amounts of real acceleration from locomotion, I'm assuming you mean consumer "slipmill" type setups where you fully run in place).
Not really. Your proprioception is accurately sensing the same as your vestibular system, which is that you are moving your limbs but you don't have forward momentum because you're not moving. Which is totally fine and indistinguishable from normal in the steady state of walking in one direction at a constant speed as on a regular treadmill, but the illusion falls apart the second you try to turn or change speed, as you'll want to do on a VR treadmill. And it doesn't matter how big the treadmill is, the problem will occur whenever the treadmill changes direction or speed.
If you apply galvanic vestibular stimulation to try to "correct" at least the vestibular sense, you will lose your balance and fall over, because you can't fool physics. Even if you're prevented from falling by a harness it won't feel right. The disagreement between your vestibular and proprioception senses will be uncomfortable.
