Buffering VOD is indicative of the internet speed not being sufficient to support the streaming video, but...
The V6 doesn't connect directly but shares your home broadband via your homehub. If thats ok, then the broadband in your area is evidently fine.
The TiVo has an inbuilt connection separate from your home broadband, and in the past it used to use a separate section of the VM broadband network. (Docsis 1 vs Docsis 3) - I'm not sure if this is still the case.
It sounds like the broadband your TiVo has, might be in congestion at peak times while the broadband that all your modems (and by extension, your V6) use, is fine.
There's no quick fix for this (unless you wanted to swap your TiVo for a V6, that's another debate), the best you'll manage is for this to be confirmed either way. If there's a network fault causing this, then there may be a quicker fix.
I've raised this post for a second opinion by the VM forum staff. It may take a day or so for them to respond, but this should certainly expedite things either way.
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I am still waiting for the promised response a week later !! As I have been with you for +20 years that's a little disappointing and clearly will need to look at going to another provider unless someone gets in touch please