• They both like to remain mysterious and unseen.
• They both have a passion for dressing in black.
• They will both kill you before you realise what's happening.
• Both are popular with fantasy and gaming geeks.
• Both are rumoured to be capable of virtual invisibility.
• Both are usually portrayed as "bad guys", but also have a significant section of sympathetic literature.
• Both can move completely silently.
• Both can easily climb sheer surfaces.
• You can't see either of them in a mirror.
• Both have pallid skin from lack of exposure to sunlight.
• Both are enhanced by black and white photography.

Can't they all just get along?

Ninja photo CC BY 2.0 Fuzzy Gerdes. Vampire photo CC BY-NC-SA 2.0 solofotones.