VOD means Video On Demand - among other things, CatchUp etc.
Streaming programmes between your 2 boxes uses the same connection to your homehub as OnDemand uses, so if one version of streaming works, there's no reason the other shouldn't. You've mentioned that you "think they're connected" and then "the internet connection is fine" on other devices - which is making me suspect your underlying problem could be the connection between at least one of your V6 boxes and your homehub, that's what then provides the internet for your V6's.
Do you get any error messages when you try to stream programmes between the boxes, or indeed when you use OnDemand? By means of investigation, the connection between a V6 and the homehub can be either WiFi or Ethernet cable, but if one box is struggling to get a reliable WiFi signal, that will be the weak point.
I'd suggest checking your V6's internet connection directly, by following Home > Help & Settings > Settings > Network, and seeing what's reported in the top-right. On any WiFi-connected box, that will include a read-out of your signal strength.
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