In the Demon the Fallen Core Rulebook whenever it talks about a demon's followers, worshippers, thralls, or vessel it always explicitly qualifies them as being mortals. Vampires are not mortals. However, there is extensive canonical material in classic World of Darkness which describes demons either having vampiric followers/worshippers/thralls, forming pacts with vampires, possessing vampires as vessels, or otherwise exploiting vampiric politics for their own gain.
Can a demon use a vampire as a follower/worshipper/thrall or a vessel? If so, can a demon form pacts with and/or gain faith from a vampiric follower/worshipper/thrall? Can a demon gain the benefits of vampiric disciplines by possessing a vampiric vessel?
- The Earthbound Kupala.
- He is revered and worshipped by the Tzimisce (to the extent that their Koldunic Sorcery blood magic discipline is at least partially empowered by their connection to and invocation of Kupala).
- He bestows oracular visions related to Gehenna upon the Malkavian prophets Octavio and Anatole.
- He is at least partially responsible for influencing the original mages that formed the Tremere to turn themselves into vampires.
- The Earthbound Nikanuuranu
- He formed a pact with the Inconnu's Council of Twelve whereby they offer him a mass human sacrifice along with some of their own blood to maintain a protection spell over their base of operations. It's worth mentioning that the wiki explicitly points out that he does not gain faith from the Twelve themselves, but instead from the mortals that they sacrifice in his name; he does however gain the benefits of each council members vampiric disciplines for the purpose of fulfilling his pact to protect them along with their haven.
- The Earthbound archdukes (particularly Abaddon and Asmodeus).
- They supposedly gain great power by being the focus of the Greek/Roman Empire's religious practices. However, Greece is largely the province of vampiric authority: Crete is controlled by the Toreador methuselah Minos until he is killed and diablerized by his child Helena, and the rest of Greece is later conquered by Alexander the Great in large part due to his mother and the Malkavian methuselah Cybele. Additionally, Rome during this time is ruled by the methuselahs of the Ventrue clan (one of whom, Tiamat, is possessed by the demon Drakonskyr) which form the Eternal Senate which later becomes the Inconnu. Additionally, the Punic Wars between Rome and Carthage in this time period are one of the earliest recorded massive conflicts between the those vampires that would later comprise the Camarilla and those that would later make up the Sabbat.
One commonality that I observed from the above examples is that all of the demons save for Drakonskyr are explicitly called out as being Earthbound demons as opposed to Fallen demons. An answer which uses this observation to conclude that only Earthbound demons can make pacts with and/or gain faith from vampiric followers/worshippers/thralls should provide some additional rationale for why that is so (citing some canonical source is ideal but not strictly necessary) in order to be acceptable, as correlation is not the same as causation.