Short answers, pulled from the story.
Vanth was discovered by Michael Brown and Terry-Ann Suer using Hubble Space Telescope images taken on the 13th of November 2005. Brown and Suer reported the discovery to the Central Bureau for Astronomical Telegrams, which announced it on the 22nd of February 2007.
Vanth has a diameter of approximately 443 km, making it roughly half the diameter of Orcus. It is the third-largest known moon of a trans-Neptunian object, after Charon and Dysnomia.
Vanth is the name of the winged Etruscan psychopomp who guides souls of the dead to the underworld. The name was first suggested by fiction writer Sonya Taaffe and officially approved by the IAU, with the naming citation published by the Minor Planet Center on the 30th of March 2010.
Like Pluto and Charon, Orcus and Vanth form a binary system where both bodies orbit a shared barycenter. Vanth's mass is 16% of Orcus's mass, which is greater than the Pluto-Charon mass ratio of 12%, making it the most massive satellite relative to its primary among all known planet and dwarf planet systems.
Orcus and Vanth revolve around their shared barycenter every 9.54 days in nearly circular orbits, with their centers 9000 km apart. Vanth's orbit is inclined at 90 degrees to the plane of the Solar System.
Vanth is believed to have formed from a giant impact early in the Solar System's history, likely before Neptune's outward migration about 4 billion years ago. Hydrodynamic simulations by Sota Arakawa and colleagues in 2019 suggested an impactor traveling near Orcus's escape velocity struck at an oblique angle greater than 45 degrees, leaving a large fragment that eventually became Vanth.