This section of the code looks wrong:

                    if (vline_data & 1) { // only draw if pixel set
                        if (0 <= y && y < self->height) { // clip y
                            uint byte_pos = x0 + self->stride * ((uint)y >> 3);
                            if (col == 0) {
                                // clear pixel
                                self->buf[byte_pos] &= ~(1 << (y & 7));
                            } else {
                                // set pixel
                                self->buf[byte_pos] |= 1 << (y & 7);
                            }
                        }

If the pixel is not set, it should surely clear down the pixel in the framebuf as we don't know its prior contents. Further, if we happen to be drawing with col==0 to a part of the framebuf which is already zero, then nothing will be drawn, because in that circumstance the code never sets a pixel.

I hope I'm right on this observation from reviewing the code: I haven't had the opportunity to try it since my SSD1306 was DOA.