Virgin Atlantic are constantly assessing potential new routes, but to my knowledge, Birmingham does not feature on the horizon at the moment. The next UK departure airport will be Glasgow, and that will be for only 6 departures this season!
The runway length at Birmingham is indeed limiting, and ground manoeuvring for A340-600 aircraft is extremely difficult, which constrains Virgin's options somewhat (the A340-300s will be history before long). However, the airline is highly likely to become a large twin operator in the future, and airports like Birmingham may become more attractive once those aircraft are in service.
While there are no current plans that I'm aware of, that does not mean that Virgin will never operate from Birmingham.
Scroggs