Madewira

First thing, adjust your photo's level using image editing software such as Adobe Photoshop until its brightness and contrast looks fine. This step will easing you to find and trace your photo's outline.



After that, pick about three color samples: light shade color, normal skin color, and a darker shade.



Let trace the outline and change each shape you are created into 50% transparency, so hinting the overall photo's outlines and details would be painless. Leave the acne intact, no need to trace it



Be sure to pay an extra attention when tracing eyes, nose, and lip. These parts are very essential, a small difference would make your illustration significantly looks strange.



And then, use the color samples above to distinguish the space between eyes, nose, lip, and ear. Use the darker shade as a shadow, and the lighter ones as a skin reflection. Put some blur on, and your vector illustration is just finished