Here is the article on the disclosure of the technology that made vtubing as it is today possible
>“Live2D Euclid” technology creates 3-D models while retaining the essence of a 2-D drawing
https://soranews24.com/2015/01/15/live2d-euclid-technology-creates-3-d-models-while-retaining-the-essence-of-a-2-d-drawing/amp/

From that point, MMD “riggers” flocked to Live2D (much easier than hand moving seven hundred “bones” in a MMD 3D mesh)