According to Tony Augarde's Oxford Guide to Word Games, univocalics are writings that make use of only one vowel. This is apparently an old form of word play, as Augarde gives several examples from the nineteenth century, including one that dates back to 1824.
"Mary Had a Univocalic Lamb,"
Word Ways: Vol. 19
, Article 8.
Available at: http://digitalcommons.butler.edu/wordways/vol19/iss3/8