What is Vermeil?
From Wikipedia: Vermeil, pronounced vərˈmeɪ is a combination of sterling silver, gold, and other precious metals, commonly used as a component in jewelry. A typical example is sterling silver coated with 14 carat (58%) gold. To be considered vermeil in the US, the gold must be at least 10 carat (42%) and have thickness equivalent to at least 2.5 micrometres of fine gold (a 12 carat [50%] plating would need to be 5 μm thick). In the US, sterling silver covered with a base metal (such as nickel) and plated with gold cannot be sold as vermeil without disclosing that it contains base metal.
The US Code of Federal Regulations 16, Part 23.5 defines Vermeil: "An industry product may be described or marked as 'vermeil' if it consists of a base of sterling silver coated or plated on all significant surfaces with gold or gold alloy of not less than 10 karat fineness, that is of substantial thickness and a minimum thickness throughout equivalent to two and one half (2½) microns (or approximately 1/10000th of an inch) of fine gold."