You could certainly specify that standing play is required and have checks for that. I tend to get super sick when I do standing VR, especially with smooth turning and smooth locomotion. But I love VR, so I do what I can, and that means stationary seated play with snap turning. As long as I'm seated and the framerate is consistently high, smooth locomotion is fine. I think your game could be played seated though, without too much modification, just a setting for height calibration probably, and maybe allow a crouch with a button or pulling back on the right thumbstick like in Bonelab (and my entry).