I don't think you would have any grounds to get a refund from VHI for having a second health insurance policy with another insurer.
From VHI's point of view, the policy holder, i.e. your father if I understand your post correctly, should have removed you from his policy when you took out another insurance contract, or at least at the first renewal afterwards.
Obviously you can be removed from his policy going forward, or at least from the next renewal, but I don't think VHI would be obliged to refund any premiums already paid.