With the MED1 form you can claim back at the top rate (42%) what you paid out ie the full prescription and doctors fees etc per your receipts less of course the first €125/€250 of receipts.
With VHI it varies according to your plan. Read your Guide to Plans and Rules booklet.
For instance you may be able to claim only €20 per GP visit and €51 per consultant and no prescriptions at all less of course the first €500 for a family.
My own experience is that if you have very high outpatients charges you should claim on the VHI and then claim the full amounts on MED1 reducing them by the refund.