I normally work more on C# Unity and Networking, but I'm researching VN/VN-Style story telling for a new potential project.

Few questions:
A) Would Vectorial sprite animation work with Naninovel?

B) Is Vectorial Sprite animation the correct terminology?
(I mean Fluid sprite animation that seem like a mix of stretching the sprite and bone animation. Kinda like some vtuber have, when they don't use 3D motion capture.)

C) Think it would be suitable for "escape room/dungeon" storytelling, like The Dark Room? (I think it would be)

D) Does Naninovel come on sale on the website as well as the Unity Asset Store? (I was interested in the asset at half price)

It's called skeletal animation. There are multiple options: Live2D, Spine (both are supported out of the box by Naninovel) or a custom solution with generic, layered or custom Naninovel actor implementation; find more about each option in the characters guide: https://naninovel.com/guide/characters

I'm not familiar with the game you've mentioned, so can't tell for sure; if it's similar to visual novels (for which Naninovel is designed), it should fit.

For the time being, we are not planning sales on the website.