How to make vector portrait  

The tools you'll need for this tutorial. Computer:mouse:adobe illustrator and adobe photoshop.
I can tell you some good tricks to help you to do you portraits: at least from photos.
Open the original photo on photoshop File>Open, so apply the filter Filter>Posterize, this filter will divide the color in "steps" so you can see more cleary the color divisions. Set the desired steps level and save the picture.
Open this pic in the adobe illustrator and used the pen tool to draw the color blocks..The final result will be very realistic. You should start with simple pictures:with less element, maybe facial picture. With more experience you can go to hard challenges.