If your issue is how to connect two phones at the same time, you would just need a double splitter/adapter connected into the existing VM adapter from the back of the hub.
If you want to have the retro phone and wireless base station in different locations, then you would need an extension kit with a splitter/adapter on the end which plugs into the existing hub adapter and at the other end a regular telephone socket.
The above might be limited, however, by whatever the hub's capabilities are to ring more than one device at the same time.
I have tried using a splitter into the adaptor supplied by Virgin. Unfortunately, the lines don't like this option and when I tried phoning home from my mobile it stated I was unavailable and to leave a message. I tried them individually through the splitter and the wireless base phone worked fine. The retro phone didn't (cut out when trying to dial my mobile number). When both are plugged into the splitter neither work.
Both phones work when put directly into the adaptor.
It's a mystery to me but I hope someone with more knowledge than me has a solution (couldn't see if Virgin provide a double socket adaptor)
When you say 'retro' phone, do you mean a retro-looking phone (but with modern electronics inside, push button keys to dial etc.) or do you mean a genuine retro phone (old electro-mechanical dial etc.) which has been converted in some way to run on a modern touch-tone phone system?