First you will need to learn the subdue skill on the skill tree. Once you have it press when an enemy is close to you to subdue him, then run to him and press and hold A to feed on him. As this action refills your vitae you will probably use the skill very often during the game, but for the achievement you only need 5 kills.
While this is the easiest way to subdue enemies, they also get dizzy if you do not finish feeding on them. Finally, if you level up your shadow kill, the enemies standing close to the one you shadow kill also get subdued and can then be fed on.