That "vine-wreath" backmark usually has the word "Plated" at the top in between the two ends of the wreath. It dates from the 1790s to about 1830. At the time, the word "Plated" in a backmark always meant silver plated. That confirms your eyes are seeing some silver on your 1-piece brass flatbutton's front.
Also... at that time, it meant the famous Sheffield Plate, a process invented in Britain, in which a thin sheet of silver was bonded onto a thicker sheet of copper, or brass.. (see the Wikipedia entry.) Electroplating had not yet been perfected.
By the way... just a friendly small correction to what you wrote about it... there is no such thing as "silver gilt." The word Gilt always meant gold plate.