Vireton and the Verlaine-Rajon “pairing”

Verlaine and Rajon worked together on Sonnets et Eaux-fortes , a luxurious volume conceived by Burty. What were the circumstances in which their collaboration came about? Who is the person behind the name “Vireton,” the third contributor to the book? And what was the significance of the poem “Le Pitre” in 1868, then later in the context of Jadis et Naguère?