I point out that on Earth normal animals eat a great variety of foods. Some species have very specialized diets and others have very generalized diets.
But "all" species seem to need some salt in their diet. And some animals sometimes need additional salt not found in their normal diet.
A
salt lick is a salt deposit that animals regularly lick.
https://en.wikipedia.org/wiki/Salt_lick_(disambiguation)
A
mineral lick (also known as a
salt lick) is a place where
animals can go to
lick essential
mineral nutrients from a deposit of
salts and other
minerals. Mineral licks can be naturally occurring or artificial (such as blocks of
salt that farmers place in
pastures for
livestock to lick). Natural licks are common, and they provide essential elements such as
phosphorus and the
biometals (
sodium,
calcium,
iron,
zinc, and
trace elements) required in the springtime for
bone,
muscle and other growth in
deer and other wildlife, such as
moose,
elephants,
tapirs,
cattle,
woodchucks,
domestic sheep,
fox squirrels,
mountain goats and
porcupines. Such licks are especially important in
ecosystems with poor general availability of nutrients. Harsh weather exposes salty
mineral deposits that draw animals from miles away for a taste of needed nutrients. It is thought that certain fauna can detect
calcium in salt licks.
[1]
https://en.wikipedia.org/wiki/Mineral_lick
So I can imagine that planet M-113 had a ecosystem that was slowly destroying itself by scattering and un concentrating salt. Possibly plant life didn't concentrate salt, and herbivores couldn't get enough salt from their plant diets, even though carnivores possibly could get enough salt from their diets. So animals would get additional salt from natural salt licks.
But possibly the ecology and geology of planet M-113 didn't form new salt licks as fast as animals used them up. But the intelligent beings of planet M-113 could easily find or make sufficient salt for themselves, their livestock, and any wildlife they chose to help survive.
But when the M-113 civilization fell, the M-113 people, their livestock, and any remaining wildlife couldn't find salt that was concentrated enough in the environment, except from within the bodies of animals.
The situation is complicated by the question of whether the Salt Vampire was an M-113 native or a being that preyed upon them for salt. Possibly an enemy introduced the Salt Vampires to planet M-113 to attack the natives and cause their civilization to fall.
Considering how much salt the Salt Vampire sucks in the short episode, it might be a being with a diet largely, but not entirely, restricted to salt. In that way it would be somewhat similar to a vampire bat that lives on blood. Since vampire bats are tiny, a single one won't take enough blood from the large animals they drink from to harm them. Being many times larger than a vampire bat, the Salt Vampire would need to drink a lot of salt, enough to kill an animals the same size as it.
The Salt Vampire would probably also need to eat other foods for balanced nutrition. Most animals need only a little pure or extra salt in their diet, while the Salt Vampire seems to somehow eat mostly salt in large amounts and apparently needs only a little extra food - which it is not seen eating so we don't know about that part of its biology.