Before KMW, the infernal penalty was paid to avoid tapping the infernal vampire in the untap phase. Now, as of KMW, the infernal penalty is paid to untap the infernal vampire (who now doesn't untap as normal).

So if the vampire is untapped already at the start of your turn (either because he didn't tap or because he untapped some other way via Majesty or Change of Target or the like), you don't have to pay to keep him that way.