@burnbrunet This feature is for wordpress.com instead of wordpress from wordpress.org which you are using.
@dsinclair
Take a look here:
https://developers.google.com/vr/concepts/vrview-web
You can try the iframe example, and embed an iframe in the gridder by using "+more"->"+html":
<iframe src="//storage.googleapis.com/vrview/2.0/embed?video=link/to/video.mp4&is_stereo=true">
</iframe>
If you want to use javascript for the initialization as explained in the link, then please read this before: http://laytheme.com/documentation.html#custom-javascript