Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Vannĭus, ii, m., a king of the Quadi in the time of Tiberius, Tac. A. 2, 63; 12, 29 sq.
Hence, Vannĭānus, a, um, of Vannius: regnum, Plin. 4, 12, 25, § 81.