I have been trying to get a reward seat in the Summer holidays for 4 years. Basically if you are stuck with school holidays Virgin rewar miles are worthless.
As others have said you can get a Miles plus money seat which will save you around £70 but you will never be able to get a reward seat in school hols.
This is one of the reason we have given up flying with Virgin as the service was so bad it was probably the only plus side, but actually proved to be worthless.