There is no such thing as "today's data" in real-time cubes. As soon as you compress the cube, data you entered today is mixed with historical data in one record.
If you don't compress the cube then you can create a Customer Exit variable that will lookup BW tables to find out what was the first request loaded today and restrict to requests bigger than that value. Or you can maintain that value in some table when you load from BPC to BW.
This is a slippery slope because it will be a ton of exceptions, like you were unable to load data to BW last night for some reason or that load failed etc.
I'd suggest better restricting it on some data, like current months from BPC, all previous from BW.
Theoretically query on Virtual provider is slower than on base cube, but in reality you don't see that difference if query is built correctly.