People who drink blood-yes, it's possible, but while the nutrient supply is high enough, the caloric intake isn't enough to support a human without killing someone every couple of days, which would be noticed. It would help support an anemic, but it couldn't be the only source of nutrition.

The living dead-impractical. The nervous system breaks down too soon after cardiac death to be revived functional, and replacing it would be mind-bogglingly complex.