It doesn't say online that it applies to the first V+ only - but it also doesn't say that it applies to multiple V+ boxes either
Personally I do get annoyed that whoever puts the Virgin's legal terms together don't make them as clear as they could be.
If you do want to argue it further you can always fill in the complaint form here
www.virginmedia.com/contact
But I don't think you are going to get any joy at all