The terms VS and VSOP mean the same thing in Cognac and brandy. They indicate age and infer quality. Cognac and brandy are distillations of wine. The Cognac region of France produces Cognac. Brandy is distilled anywhere in the world.
When VS appears on a Cognac label it means Very Special. Three stars (***) can also be used for this designation. It means the Cognac is a year and a half old.
VSOP on a Cognac label stands for Very Special Old Pale. Reserve is an alternate designation for VSOP. To be VSOP, Cognac must be four and a half to six and a half years old.
VS and VSOP are marketing designations that imply quality. Producers blend Cognacs and brandies by mixing an older vintage with a newer vintage. Single-vineyard producers use the approach of blending old vintages together. This gives a consistent and superior taste. Single vineyard and Estate Bottled mean the same thing and indicate the highest-quality Cognac.
References and Resources1st Cru: All About Cognac Aging
Le Cognac: FAQ du Cognac
1st Cru: How To Read A Cognac Label