If a vampire is supposed to survive exclusively on blood and consume it without vomiting, it makes since that they must adapt to a specific digestive process.
Looking at leeches and vampire bats, the idea is plausible.
In order to store blood for slow digestion, leeches secrete an antibiotic into its digestive system to prevent bacterial growth and to delay decomposition and then a special enzyme is secreted.
Vampire bats are sanguivores and mammals, which are closer to a vampire than a leech. The blood the bats ingest contains more iron than they need, so it has a special digestive process to excrete the excess. Basically, a mucous membrane along the intestinal tract acts as a barrier to prevent too much iron from getting into the bloodstream. There are also numerous capillaries that work to absorb the blood more quickly into the body than humans can.
SO, in order for a vampire to be able to survive on blood alone, much less large quantities, their bodies must undergo some adaptive process for the proteins, enzymes, and nutrients to be properly absorbed.