Hello, if tf isn’t available in GL ES 2.0, how can I achieve reading back transformed vertices? How would I connect a second Vertex Shader to the first One?
You don’t.
OpenGL ES is less functional than desktop GL.
The best you can do is try rendering to a texture, then using glReadPixels to copy it to a buffer object (assuming you have PBO support), and then use that buffer to draw from. But that’s the best you can do.