This short story would obviously have been written no later than the 1980s, though in my recollection the style was more reminiscent of the 1950s or early 1960s. Darker than a typical Asimov, but fitting his style.
A Soviet researcher is working on a model of the food energy consumption (and hence efficiency) of the Soviet Union. I don't recall the details, but he has statistical data for how much food is produced, consumed, wasted, etc.(inputs) and how many people are doing what activities and hence their energy expenditure.
He finds there is a statistically significant discrepancy between the amount of available energy and the amount of energy usage that can be accounted for. He carefully and thoroughly examines the data and finally is forced to conclude and unknown factor is parasitically draining a percent or two of total human energy.
He writes up his findings and they rocket upward through the bureaucracy, leading in short order to his being asked to present his paper to the Party bosses or the Politburo or some such august assembly
all of whom are vampires.