In times when it was common to dig up the dead and stake it for fear of a vampire was accepted, there was also a large problem with people being buried alive. What if someone was buried alive, brought near to the point of death, and like a monk in meditation, use only the minimum to survive for days. This would more than likely cause astral travel, and thus, the astral body could feed from the energy on the locals, supplying just enogh energy to keep the faint vital functions alive. At least until the supposed corpse's heart deepthroats some wood.

I heard that they used to bury people in shallow graves and that upon reviving from a coma the "corpse" would sometimes fight it's way half out of the grave and die in the effort or exposure (if they weren't ill they would not be in a coma. People would find the dead half out of a grave and think "Vampire." I heard that the stake was because Undeath is an unnatural condition and unnatural weapons of metal were ineffective. Wood being a natural substance would strike the vamp dead by connecting it with physical reality where it could not survive. The heart being the natural place to connect.