MLP 93-I-5-1 was discovered in 1989, by Eduardo Olivero, Daniel Martinioni, Francisco Mussel and Jorge Lusky, at Cape Lamb of Vega Island at the edge of the Antarctic Peninsula of James Ross Archipelago (northernmost part of Antarctica).

MLP 93-I-5-1 was collected from an area of 3 square meters in a semi-articulated state, at locality "Assemblage 10", which belongs to the middle section of Cape Lamb Member of the Snow Hill Island Formation, dating to the early Maastrichtian stage of the Late Cretaceous, about 72 million years ago.

The specific name honors the Argentine paleontological technician Juan Jose Moly, for participating in 17 Antarctic field trips and in the collection of the holotype.

In the resultant topology, Vegasaurus nests with three other Late Cretaceous South Pacific elasmosaurids, being Morenosaurus, and two species from the Weddell Sea region, the aristonectines Aristonectes and Kaiwhekea.

[1] Eromangasaurus australis Tuarangisaurus keyesi Elasmosaurus platyurus Libonectes morgani Callawayasaurus colombiensis "Libonectes" atlasense Hydralmosaurus serpentinus Hydrotherosaurus alexandrae Aphrosaurus furlongi Terminonatator ponteixensis Thalassomedon haningtoni Styxosaurus snowii Albertonectes vanderveldei Futabasaurus suzukii Mauisaurus haasti Wapuskanectes betsynichollsae Morenosaurus stocki Vegasaurus molyi Aristonectes parvidens Kaiwhekea katiki