The "tattoos" are model numbers. Some companies (such as Crypton Future Media) put model numbers on their Vocaloids (exp. Hatsune Miku is "CV01", Kagamine Rin/Len are "CV02", etc. Which stands for "Character Vocal.")
Not all companies who develop Vocaloids do this, only a small number of Vocaloids actually have tattoos.
Not all Vocaloids do have tattoo's. Sometimes it is just part of the design while with other like the Crypton Vocaloids it is to show what number they are in their "Character Vocal" series. The only ones with confirmed Tattoo's are the following:
Miku Hatsune Len/Rin Kagamine, Luka Megurine, Sonika, Lily, and Mew
Also note that Big Al Sweet Ann Tonio and Prima also have tattoo's but they are on their Taiwanese design which are not considered official.