A vampire system is a system of wearable objects (usually a HUD and some accessories), created for a vampire roleplay game. This includes, but is not limited to Bloodlines.
Reason to study
Demography is a part of Geography that studies populations (residents for Second Life): structure (based on ethnic, age, gender, religion or other criteria), density (for a specified area) and fluctuation (for a specified period of time). Since the creation of Bloodlines, a big segment of population has joined vampire systems, so much that they are a large sector of grid population.
A vampire system is made for interactive use. Some of them are made for combat and some only for roleplay. Despite naming them vampire systems, they might be created for other types of mythological creatures: lycans, ghosts, dragons and not only.
What is common to all (or almost all vampire systems) is the use of a large database, usually hosted on an external server. This allows a good way to store ratings for all players.
A third feature of vampire systems is the use of blood or another source of energy (strength, lumens, humanity and others, usually referred as vital fluids). Vital fluids might be collected from other players, might be purchased from vendors and are important in rating. They are consumed during roleplay or they can become fuel to keep the player alive (in this case, they are slowly consumed). So, a player will need to get vital fluids to stay alive and to get ranked.
The examples are listed in alphabetic order, to avoid discrimination.
- Bloodlines Vampire System 
- Empower Immortals Vampire System 
- Hunger (First Blood) Vampire System 
- Netherworld Vampire System 
- Progeny Vampire System 
Other vampire systems might exist.
Pain Of Death is not a vampire system , but a tool created to detect and attack vampires.
The following vampire systems appear to be shut down:
- Vitae Vampire System 
- Sangre Vampire System.
A vampire system requires an external database, where all statistics are displayed. However, you might find on marketplace many HUD devices that can work exactly as (or even better) then a vampire HUD. It can attack others and create massive spam around, but it will not grab souls from unregistered players. There are many examples on marketplace: , ,  or .