I don't know if it was in CoS, but in Van Richten's guide, the subtitle for Barovia is: Domain of the First Vampire.
There is also the line: The nature of his bargain with the Dark Powers was revealed, and Strahd became the multiverse's first vampire.
I largely am ok with Strahd being the first vampire, just because vampirism is largely an inefficient form of undead... compared to being a lich or mummy lord, vampirism is almost more of a mistake. Warhammer Fantasy essentially says this outright, I believe there Neferata becomes the first vampire by mistake when trying to achieve immortality by mimicking Nagash's research.