This relic of the Vault was probably used to contain some sacred sacrament. The holy number 13 is emblazoned on the side of this precious link to the past of the Chosen One's people, and their hopeful future.
This flask is given to the Chosen One by the Arroyo elder once they complete the Temple of Trials. When they find Vic in the Den, he will suggest they show it to Ed in Vault City. When done so, Ed will mark Broken Hills, Gecko, Redding, and New Reno on the map. Go find that G.E.C.K.!
Three more can be found in a desk in Vic's home in Klamath, but they have no use.