I don't know. I blew up the image and there is a texture to the skin.  It's not all gradients. The outline mode doesn't match all the color changes.

Corel's longtime Hedy Lamar box art was clearly vector art, but incredibly well done. Some of these may be a mix of Illustrator and Photoshop, but the most realistic don't appear to be standalone vector art. Without seeing an example showing the intermediate steps, I'm going to remain skeptical. Vector art, at its best, looks like airbrushed work. I haven't used a vector art program in a while, but getting photographic quality to the extent of this image isn't possible with its blurred distant hairs and bumped skin textures.