Vermeil means gilded silver, that is, a body of sterling silver with a layer of gold over it. Montblanc used this construction for Solitaire models for many years. It does have a drawback: silver tarnishes, and since the gold layer is thin and wears through over time at edges, engravings and the places where the pen is held, Vermeil instruments become blotchy and dark in use. Production was therefore changed over; the newer gold-plated writing instruments are made of gold-plated brass, which does not tarnish and stands up better to everyday use.
Quite wrong, on the other hand, is the occasional claim that brass is the same thing as gold-plated copper. Brass is an alloy of copper and zinc, often with a small proportion of lead; the alloy of copper and tin is called bronze and is somewhat harder than brass. For buyers the distinction genuinely matters: a true Vermeil instrument normally carries a silver hallmark such as Ag 925, weighs rather more than a gold-plated brass version and is valued more highly on the collectors' market. Sellers' descriptions that run Vermeil and gold-plated brass together should therefore be read with caution.