Originally the steak through the heart thing wasn't fatal, it was used to "pin" the vampire into it's grave preventing it from rising again. The body would then have a brick or stone forced into it's mouth to shatter it's teeth (obvious what that was about) and would usually be decapitated (the fatal part of the endeavour, although done alone this would not suffice) which would all contribute to ensuring the creature could no longer harm people.
Like with many things in modern vampire mythos, the steak through the heart has been misinterpreted either by accident (like a sort of chinese whispers effect) or deliberately for dramatic purposes.
Other examples include;
Fatal reaction to sunlight (vampires avoided daylight because in daylight it was obvious that they were dead and their other powers were dramatically reduced making them vulnerable..don't even bring up sparkling).
Aversion to crosses or other Christian symbols such as holy water (Christian propaganda inserted into the myths for obvious reasons)
Fatal reaction to garlic (something to do with the purifying attributes given to garlic, although originally the reaction was not fatal)
Fatal reaction to silver (again due to the perceived purity of silver, the same reason for vampires casting no reflection in mirrors which were made of silver, although there is some evidence in some myths that a silver bladed weapon was needed to decapitate a vampire but silver itself wasn't fatal).