Here are my 2 vector portrait images of Trey Songz:
1. Open an image of artist in Photoshop
2. Add a smart blur to simplify the image.
3. Then posterize the image, changing the number of levels to around 2 or 3.
4. Next open up the image in Illustrator.
5. Create a separate layer for each colour in the image.
6. Choose a layer to work on a lock all the others.
7. Use the pen tool to draw around all the areas of your chosen colour.
8. Repeat this step for each layer/colour.
9. Finally change the fill colour of each layer you have traced over with the pen tool to get this effect.
For this image I used the same technique as the image about but when posterizing the image I used a lot more levels, so the image is more detailed. Therefore when I created the image in Illustrator i had to use a lot more layers (one for each colour).