Finally got the VR900 working by complete accident.
The Vodafone router was acting up so I power cycled the router and ONT box and for some unknown reason, the router reset itself back to the default WiFi name for a few minutes before dropping out entirely. I called Vodafone and they said they had to configure the router manually from their side as it wasn't doing it automatically. After 90 minutes of hard resetting, power cycling, unplugging and waiting, flipping the LAN cable around (I suspect he was getting desperate by this point), etc, it still didn't work. His final suggestion was to unplug it for an hour and then try again.
I asked again for the PPPoE details as it was clear he was out of ideas and his shift was finishing. He gave me the dsl part and password, which matched the one I received on the 3rd April (which didn't work then), but it was missing the "@broadband.vodafone.co.uk" part. I plugged in the VR900 since it was still setup with those details (including the
@broadband.... part, no cloning of MAC) and it connected immediately.
I'm guessing that's why the call was 90 minutes long with no result, he was using the wrong login. No wonder Vodafone get a bad rep, if you got weak computer skills, you're in for a bad time when things go wrong as their hardware is poor and tech support have minimal training.