I am looking to extract the polygons on this website http://goo.gl/vXhDSG
I am not sure how to do that as I can't see any references to a geo JSON file.
I can see that they are using CartoDB http://www.cartodb.com/
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You can't. If you look at the source, you will see it is built from a CartoDB visualization, namely, https://nbnco.cartodb.com/u/nbncoadmin/api/v2/viz/6978447a-bb2b-11e5-86d3-42010a14800d/viz.json, which contains all the information necessary to built that view. However, if you go to the network tab in your browser's developer tools window, you will see lots of tiles, eg:
which looks like:
From this, you can deduce, that the vectors that you see in a CartoDB visualization are generated server side an png tiles, so there is no way you can get the raw vectors, unless they are available publicly from this organizations CartoDB page. If you go to this organization's CartoDB page, you will discover that the data are not public.
This is how CartoDB can handle large amounts of vector data quickly -- it gets sent to the client as a server-side generated raster -- while keeping the raw data secure, if the provider wants to keep it private.