It rarely happens but yes it’s possible for a Demon to possess a Vampire. The reason it’s rare is due to demons generally look upon vampires as being way beneath them. There’s also the fact that the human souls of vampires are already corrupted, and in essence, owned by Hell. However, there are vampires who fight for justice and protect humans who might be potential targets for the forces of darkness. Demons are always in the business of bringing everyone they can into the fiery shadows of Hell! Since vampires have demonic DNA they are conveniently compatible vessels for a higher dimensional demon seeking physical form without having to fight the human soul, their Guardian Angel, and the general forces of Heaven. Some vampires might consider it an honor to be the vessel for a denizen of damnation while others would use their strong willpower, and psychic abilities to repel it. Often such possessions are used as a last minute emergency vessel or the demon decides the vampire is in the perfect place to partake of a diabolical device to further darkness on Earth!
Ultimately a demon would have to get permission from the original demon who spawned the first generation of any given vampire line. The dark spirit of the Father or Mother Demon is connected to every single vampire in each successive generation. He or she technically possess the human soul of the vampire and is its keeper in Hell. That being said the Parental Demon is the only one who can possess any of its vampires instantly without permission. The vampire would be helpless to fight it off as it’s soul is already intertwined with the demon! High-level Arch-Demons and the Devil himself would also be able to possess a vampire with ease, and would not require the permission of the Father Demon. This is due to the Archdemons being the quintessential Fathers Of Hell itself! Although depending on the demon’s power, and importance permission would be requested as a sign of unholy respect.
Since vampires are generally considered 1/4 demon to a degree it’s plausible that a fully possesses bloodsucker would have its DNA, and body structure altered enough to be considered a Demi-Demon. Although, this depends on the generation of vampire along with its age. Higher generations of vampire, and ancient ones have the most paranormal powers. As a Demi-Demon it would no longer crave only blood but the entire human body bones, and all! This doubly monstrous manifestation would cease once the demon left the vampire’s body. Like human possession, the vampire’s body would eventually weaken from the demonic possession due to the powerful burning dark metaphysical energy a demon exudes! The paranormal vampire body would last a lot longer, and the whole human meals would aid in an extended stay but the body would eventually decay under the supernatural stress. Only a demons natural physical reptilian like form can sustain it in this Universe. Luckily Angels actively prevent such manifestations that require so much focused power that it sets off alarms throughout the Heavens! So it must leave when the vampire dies if it takes it to that extreme. There are exceptions like the Devil’s human-like form that has been in existence since Halloween 2013. It was created within a magical metaphysical maelstrom of mysterious Moira.
It’s hard to pinpoint the signs of vampire possession vs the blatant signs of human possession. The most obvious is a constant red glow in the eyes. Such crimson illumination is usually displayed only when a vampire is extremely angry or serious jacked up on sanguine energy from gorging on blood! Total blackening of the entire eye, including the whites, is another sure sign. There is also often an additional set of fangs, a greater resistance to sunlight, and the facial features grow more monstrous when it’s agitated. If you’re dealing with a good vampire then negative changes in behavior will be a warning that something isn’t right. Use of religious artifacts along with an Exorcism might aid in expelling the demon but you run the risk of killing the vampire! Often in these cases, use of Witchcraft that calls upon the Earthy Nature Gods & Goddesses has a better chance of success due to their more neutral nature in the battle between Heaven, and Hell. If the demon is exorcized expect the formerly possessed vampire to suffer major setbacks in his or her quest to grow closer to humanity, and shun the darkness!