Well I think I'm the only person who has replied who has flown both. I've done BA WTP to and from SYD and VS Prem Econ to UVF and from LAX.
I like both airlines equally and have no "grump" against either. Comparing just those 2 classes I can easily say that VS Prem Econ is more spacious all round than BA WTP. It's not just a legroom or service thing - pretty much the same on both carriers.
It's the actual seat which makes the difference. VS will lie flatter (ie: recline more) than the BA seat & is wider.
Now I'm no giant or fatty but I just could not get comfy or asleep in the BA seat. It's as simple as that really. If I was presented with the choice of WTP against Prem Econ it would always be the VS flight.
Hope this helps.
cheers
FF