In theory, but in many versions of vampire lore vampires have some sort of issues with traditional digestion. There are multiple ways vampires are shown to feed, with the mechanism of action often partially or completely obscured, but two common examples are the literal “blood drinker” that swallows blood and only uses their teeth to create wounds- and the more consistent and logical “blood sucker” wherein the fangs are used to suction blood and often Can also inject some sort of substance- possibly vampire blood- used to “turn” chosen targets.
The “blood drinker” would most reasonably have some method of digestion- most likely a modified intestinal system, the blood sucker may not have an intestinal system at all and instead the fangs may connect directly to the circulatory system and/or a special organ or modified organ for processing blood.
Using the host digestive system to digest blood seems dubious based on most vampire lore, the reasons being numerous but two large tells relate to the classical powers of many “vampires.” The majority of vampire lore shows vampires to have extremely rapid healing, and the be able to heal by drinking blood. It is possible but unlikely that intestinal digestion of blood would provide the near instantaneous effects often described or shown. That one is weak, as there are numerous non “magical” possibilities to explain that away- but a major factor is the general durability of vampires and the fact they are often very difficult to kill or must be killed in specific ways. If the intestines were used for blood digestion and blood powers vampires healing abilities, it would seem logical that a way to kill a vampire would be to wound their intestines when they are in a weakened state or state that impeded healing. With their digestive systems injured and insufficient energy to heal them,
they could in theory be killed via starvation or permanently prevented from full recovery as without the ability to digest blood they could not replenish the energy required to heal their digestive systems and thusly would in theory eventually starve.
A common lore of vampires is that destroying their heart would kill them. This i feel supports the “sucker” theory as of the fangs are like syringes and draw blood in for use, either directly to the circulatory system or for special processing and digestion via some blood stream based mechanism, not only would that make the generally very quick reactions often observed to blood ingestion in vampires more reasonable, but it would explain why the heart is often used to kill them. The heart itself in this example becomes a critical part of digestion.
Wether the heart simply still acts as a pump and the vampire merely takes a victims blood into their own blood stream and circulates it to “bathe” their cells in nutrients, then feeds again when the nutrients in the blood are exhausted or toxins of some sort have built up to certain levels; or wether the heart is somehow converted into an organ to process the nutrients in blood when a vampire transforms- the classical and popular conception seems to indicate that vampires, or those or a certain sort, might most logically evolve to “suck” blood and use the heart to actually gain the nutrition from it.
Of course there are countless stories and myths and legends and such of vampires, and the specifics of their powers and weaknesses, origins, behaviors, etc. vary greatly. So I can’t say there is a singular definitive “right or wrong” when we discuss vampires in general or a theory pertaining to vitamin D deficiency. In various telling vampires cannot eat food at all, in others it makes them sick or is unpleasant for them (these both imply digestive issues due to lack of functioning GI systems or abnormal function of the GI system..) in other telling they can eat human food but gain no nutritional benefit from it. I’d say the general and popular or classical western concept of vampires when viewed in the vitamin D theory lens supports more strongly the “sucker” theory.
Now of course- if we want to play at real vampires based off all the lore and such- if vampires were “sucker types” and they only needed to suck blood for vitamin D and/or some form of self dialysis- and they could actually eat human food, at least to be able to safely pass it through their systems if not for any other value-
The vitamin D theory could explain in part the seeming absence of modern vampires. The first recorded vitamins were synthesized in the early 1900’s. Meaning that around the turn of the 20th century, if vampires merely need vitamin D but could t digest it from food sources or solids- or could only absorb nutrients via sucking liquids through their fangs directly to their circulatory systems-
In theory vampires wouldn’t need the cost and complexities and liabilities and conspicuousness of the common trope of “vampires hiding by drinking blood bank blood..” they’d just need bottles of injectable vitamins. It is possible they need more than vitamin D, and it may be similar to humans where humans can be supplied nutrition through other means than orally administered foods and liquids but due to differences in how those routes are processed or the chemical structures of the replacements etc. there can be long term health issues involved. So perhaps vampires can largely subsist on medical grade supplements but require an ocasional feeding of blood to remain healthy. For this they could use “blood bags” and due to the small quantities and infrequent nature of feeding, this would be far less conspicuous or likely to expose them or implicate them than freezers full of blood bags or international blood donation conspiracies revolving around clandestine centralized distribution.
The “blood drinker” would most reasonably have some method of digestion- most likely a modified intestinal system, the blood sucker may not have an intestinal system at all and instead the fangs may connect directly to the circulatory system and/or a special organ or modified organ for processing blood.
A common lore of vampires is that destroying their heart would kill them. This i feel supports the “sucker” theory as of the fangs are like syringes and draw blood in for use, either directly to the circulatory system or for special processing and digestion via some blood stream based mechanism, not only would that make the generally very quick reactions often observed to blood ingestion in vampires more reasonable, but it would explain why the heart is often used to kill them. The heart itself in this example becomes a critical part of digestion.
The vitamin D theory could explain in part the seeming absence of modern vampires. The first recorded vitamins were synthesized in the early 1900’s. Meaning that around the turn of the 20th century, if vampires merely need vitamin D but could t digest it from food sources or solids- or could only absorb nutrients via sucking liquids through their fangs directly to their circulatory systems-