What is the best way to make a vector portrait by Adobe Illustrator?

Within the majority of vector communities on the internet, portrait art is a popular theme.
Be it rendering from photo stock or reference free work, it’s a corner stone in vector art.
Browse through twenty five wonderful, unique portraits, be in technique or the final appearance, which may inspire you to try new avenues of vector art portrait creation.
All of the art work has came from members on deviantArt, where there is a strong vector community.

By converting it to different areas and shades of colors and tracing all the features independently.