From ONUV, on Team Vampire
If the Diseased or Thing is in the game, it is usually advantageous to give the Mark of the Vampire to a player adjacent to you, since you can claim to be the Diseased or Thing and the recipient of your mark can claim to have received the Mark of Disease or been touched by Thing (or vice versa), thus mutually supporting each other’s claim. Even if the true Diseased or Thing refutes this, doubt will have been sown, to your advantage.
As the Vampire, your first priority on waking should be determining if the person to whom you gave the Mark of the Vampire still has it or lost it by receiving one of the subsequent seven marks. Seven because a player can’t receive both the Mark of the Vampire and Mark of Fear, and a Renfield replacing the Mark of the Vampire with a Mark of the Bat would remain on Team Vampire regardless. If you determine that the player kept the Mark of the Vampire, subtly indicate that it was you who gave it to them, possibly by saying that you gave them another token, like a Mark of Clarity as a Priest, or imply that they as the Thing touched you on your shoulder or say that you received the Mark of Disease from them, giving them an opening to confirm your story.
Beyond this, Team Vampire strategy normally resolves to that of Team Werewolf.
What do you think? Leave your thoughts in the comments section.