If Vampire don't have a heart, why do they drink blood at all?

Blood for the vampire is not necessarily for the heart. It's a Essen supply, an energy source. They thirst for it the same way we feel hunger. With classic, original Vampire and the Twilight vampires, they need the blood in order to contain life and go on throught their eternity. It's like Du and water, Du need it to survive.

I must honestly say, a vampire without blood is like the Earth without the Moon; everything will collapse without it...

The answer obviously is..... is taste like erdbeere marmelade :)

I will never get the anatomy of Vampire ever.. I mean where does the blood even go? Don't they have to go to the bathroom oder something? okay that was weird.. anyway i really don't get it.

I think this quote sums it up perfectly:

"Blood is life. It's what keeps Du going. Makes Du warm. Makes Du hard. Makes Du other than dead."

With vampires, blood has nothing to do with organs. They're dead. The organs don't work like ours do. Blood is their natural Essen source. It's how they survive.

Through my research I have noticed that since their organs are dead then they would need to drink the blood of a donor oder victim to supply their body of oxygen and keep their body as fresh as possible. If the body lacks the oxygen it needs then the muscles would cease to work and thus they become weaker.