Lovecraft mentioned of a beast that's appeared several times in the Bible, this is a God of Philistine origin. It is commonly believed that Dagon was represented as half-man half-fish entity that's somewhat identified with Oannes; a Babylonian deity thats also known as the Lord of the Waves. Oannes is represented as a human from the upper part of the body, fish-like as to the lower. From this it may well be inferred that Dagon was a fish-god, as Dagon seems to have been the foremost deity of such maritime cities as Azotus, Gaza. According to an old Phoenician legend, Dagon was one of the four sons born of the marriage of Anu, the lord of heaven, with his sister, the earth.
The Bible mentioned that the Philistines took the Ark of Covenant and brought it from Ebenezer unto Ashdod. When the Philistines took the ark, they brought it into the temple of Dagon, and set it in front of the deity, and when the priests guarding the temple of Ashdod arose early the next morning, Dagon had fallen with his face upon the earth before the ark of the Lord. And these priests took Dagon, and set him back in his place again. When they arose early on the following morning, behold, Dagon was fallen upon his face to the ground before the ark of the Lord again; and the head of Dagon and both the palms of his hands were cut off upon the threshold; only the stump of Dagon was left to him. It seems that the power of the Ark had shattered the hands of Dagon; as the Ark of the Lord shall be housed before none other god. Therefore neither the priests of Dagon, nor any that come into Dagon's temple, tread on the threshold of Dagon in Ashdod till this day.
Probably God was angered by the people of Ashod for placing the Ark of the Lord before Dagon that it resulted in the destruction of Ashod. And when the men of Ashdod saw that it was so, they said, The ark of the God of Israel shall not abide with us: for His hand is sore upon us, and upon Dagon our god.
The Vampiric Dagon is however a creature of my imagination