Vintage Story is primarily written in C# with OpenGL as the graphics backend. Learning OpenGL is only necessary if you want to do special rendering tricks, and learning C# is necessary if you want to go beyond assed modding.
Here are a few learning resources that we can recommend:
For OpenGL, and Shaders
Playlist by ThinMatrix: https://www.youtube.com/watch?v=VS8wlS9hF8E&list=PLRIWtICgwaX0u7Rf9zkZhLoLuZVfUksDP
Playlist by TheCherno: https://www.youtube.com/watch?v=W3gAzLwfIP0&list=PLlrATfBNZ98foTJPJ_Ev03o2oq3-GGOS2