Virgin have a current contract with O2, until that expires no one will be on Vodafone
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Sorry, Virgin have no contract with O2. The present contract with EE ends this year and people are already being migrated to Vodafone in large numbers.
I don't know where you got your information but a perusal of the trade journals will confirm what I say.
The merger of Virgin mobile network and O2 network is some time off.