Why not just very good VR? Cannot believe it'd be more difficult than creating robots that can't be told from real humans.
Unless we achieve the ability to essentially run a MITM attack on the brain, to intercept commands from the brain and to provide it with sensory input, VR won't come close to being something genuinely deserving the name virtual reality.
There are scenarios where you are more constrained in real life (e.g. sitting in a vehicle of some sort) where g-forces aside, you can probably get pretty close with vision, sound, and some fairly basic force feedback will be able to get you to a fairly decent simulation. But I agree that anything involving running around and physically interacting with a 3D world is a lot more challenging.
Currently you can't even really walk around in a room without being incredibly restricted in terms of room layout, furniture and let's not forget you the headset with cables attached.
Last but not least even this hypothetical virtual reality is still just that: virtual. Simply the knowledge that something is merely virtual will always be somewhat of a disappointment, the same way a copy, no matter how perfect, is not quite as appealing as the original.
The concept of Westworld has an authenticity to it, that virtual reality can never hope to reach. As humans we are just weird that way.