I somehow missed the third portion of the Drain Vitality shout when going through the Dawnguard story line. I was a Vampire by the way. Could anyone please point me in the direction where they got the last word of the shout please? Much appreciated.

Arcwind Point, my friend. South of the Throat of the World. Be prepared if you haven't visited before, as you're guaranteed to encounter leveled undead and a dragon.

Alternatively, if you got it from there, there is a small word wall in the Forgotten Vale, next to the special boss encounter on the ice.