It looks like the VR button is automatically hidden when full screen is not enabled. You can see this check in VRButtonViewModel.js in the constructor.
var isEnabled = knockout.observable(Fullscreen.enabled);
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You could try to override that but I’m not sure if it’ll work if the browser is reporting that full screen isn’t supported. Might be worth looking into if there’s anything special you need to do to get fullscreen working from within an iframe.
The VR button checks if fullscreen is enabled, and is hidden when fullscreen support is not detected. So make sure you enable the fullscreen attribute that Scott linked to.