I too have noted an issue with the 'simplistic' reasoning for the existence of the Vaesen framed in the books description.
The issue, the chicken and the egg, Vaesen exist because people believe they do.... and given that only those with "The Sight" can actually see them, then arguably they exist because the Players believe they do, and no matter how long they fight of Vaesen that circle will keep coming around. As a Game concept, the applied explanation falls flat on its face.... It is both a completely predictable shock ending, coupled with an added stagnation that the easiest way to defeat any Vaesen is to convince people who cant see them anyway, that they don't exist and never did in the first place. Which should be relatively easy to do because the setting outright points out that the problem has been created by people adopting the scientific and stopping believing in the old ways. How exactly can Vaesen be acting up because people stopped believing in the Old ways, when they exist purely because people think they do?
"In the nineteenth century, Scandinavia is changed by industrialization, war, and revolutions, and new ways of thinking and understanding the world are spreading through its universities. Old truths are being questioned."
This indicates the industrialization and Science have had humans question the reality to the Vaesen existing in the first place. In fact they even say people have forgotten how to deal with the Vaesen, likely because they stopped worrying about fiction and superstitions. and yet somehow they became more powerful.
Regardless on how "Fleshed out" you feel this should be or how "vague" they want to leave it to the GMs the creationist aspect is contradictory to the entire premise and should be addressed on some level for clarity.
Not that I don't like the game, I love it.... but this is a serious Plot Hole.