When we talk about the strength of vermouth, we are referring to its alcohol content. Vermouth is a fortified wine, meaning that it has been aromatized and fortified with additional alcohol, usually brandy. The alcohol content in vermouth can vary depending on the brand and style.
One important thing to note is that the alcohol content in vermouth is not as pronounced as in spirits. Vermouth is meant to be used as a mixer or an ingredient in cocktails, so its alcohol content is balanced with other flavors and ingredients. This allows the vermouth to contribute to the overall taste profile of a cocktail without overpowering it with alcohol.