Vermeil means solid sterling silver with gold plating. The material has an unfortunate habit: after use and prolonged storage the silver tarnishes beneath the gold layer, so that blotchy discolouration shows through on the surface. Silver polish will remove it for a time, but it always comes back. This damaged the reputation of the range and was a major reason for the change: for some years now the Solitaire models with line guilloché have been made not in vermeil but in a gold-plated version with a brass core. Price and appearance stayed the same, so the changeover passed almost unnoticed; objectively the metal value changed, subjectively the sense of quality.
The barleycorn versions with their grain guilloché were dropped entirely as early as 1999, in sterling silver and vermeil alike, because demand was far lower and because a similar guilloché was used for a Limited Edition marking the company's anniversary. Two parallel vermeil lines were appealing to the same buyers in any case. Gold-plated pens were generally hard to sell in Europe at that time, which is why vermeil pieces lingered in some dealers' drawers for years afterwards. For collectors this means that the vermeil versions are rarer than their visually identical gold-plated successors, and can be told apart from them by the silver hallmark.