Going over the wikipedia article as a refresher and I totally forgot about how he (author Richard Matheson) had some cool biological explanations for the vampirism.

From Wikipedia:

Neville additionally discovers that exposing vampires to direct sunlight or inflicting wide oxygen-exposing wounds causes the bacteria to switch from being anaerobic symbionts to aerobic parasites, rapidly consuming their hosts when exposed to air and thus giving them the appearance of instantly liquefying. However, he discovers the bacteria also produce resilient “body glue” that instantaneously seals blunt or narrow wounds, explaining how the vampires are bulletproof. Lastly, he deduces now that there are in fact two differently reacting types of vampires: conscious ones who are living with a worsening infection and undead ones who have died but been partly reanimated by the bacteria.