you do pay in advance for your monthly allowance. But if when you phoned you gave 30 days notice that you wish to cancel (which I assume you did as you were given a PAC code) then on your refresh date there will be no money taken / allowance given & you will be moved to a pay as you go tariff.
However as by then you will no longer be a Virgin Mobile customer this is irrelevant and you will be under no obligation to VM.
If you want to say thanks > click 'Kudos'. Answered your question? > click 'Mark as Helpful Answer' also.