Being a vampire is hard work and dangerous
First you have to find your blood host, which (relatively speaking) would be found several kilometres between one meal and the next. You have to deal with the danger of being stopped from feeding, the aggressive way. Don’t expect your host to let you feed on them just like that. Although there are nice vampires that may regurgitate their meal to share with your if you have been unlucky that night.
The toxicity of blood itself is a danger. In the amounts you need, it would cause diarrhoea and kidney failure, not to mention the killing effects of the proteins and iron ingested. Apart from an overdose of some products, blood lacks B vitamins, needed for basic body function, and fat, a major energy source.
But don’t trust my word for it. Read it all in Susan Milius’s article in Science News for Students called ‘Sucking blood isn’t an easy life, even for vampires.’