Hmm. Well, where do we see similar stories?
Let us go back a long, long way. Back to Genesis, and to Moses...
Through Moses, God had visited a number of plagues on Egypt, which was keeping the Hebrews in slavery. Eventually, God pulls out the big guns, and decides to kill every first born son of Egypt. God tells Moses to have all the Hebrews mark the lintel of their door with the blood of a lamb, and death would "pass over" that house, to find another...
Later, we see similar barring of entry by marking the lintel. Depending where you were, marking your door with certain plants, or with iron, would keep out the various forms of "fey folk", disease, nasty spirits, what have you. The idea is still the same - the homes of the faithful, good folk are marked, and undesirable things cannot enter.
In more modern versions, perhaps the "marking" is simply assumed or understood, rather than done explicitly...
As to water - well, I expect that this stems back to the whole idea of baptism, which I've been told is not strictly a Christian thing. Simple symathetic magic, really. Water makes things clean physically, so it must also do so spiritually. Vampires, being very, very unclean spiritually, are thus particularly vulnerable to cleansing. Poor, unwashed vampires
