Ask a bartender what vermouth they're pouring and watch how they answer. If they reach for the bottle and have to check the label, that's your answer. If they recite the brand without looking, pull up a stool.
Vermouth is the hinge ingredient of classical cocktail-making. The Martini, the Manhattan, the Negroni, the Americano are all built on it, and every one of them suffers when it's wrong. And yet, in bar after bar, vermouth sits on the well shelf at room temperature, cork off, slowly oxidizing into something that tastes like a wine that has given up on itself. The bottle may be Noilly Prat, may be Martini Rosso, may be something with even less thought behind the purchase — and it has been there through multiple service shifts without anyone thinking much about what it's doing to the drinks.
Vermouth is a fortified, aromatized wine. That sentence does real work. "Fortified" means strengthened with neutral spirits to somewhere above table-wine strength — more than a Burgundy, well short of a whiskey. "Aromatized" means botanicals — roots, barks, citrus peel, wormwood, any number of other ingredients depending on the house — have been macerated through it. Each producer keeps their botanical formula private, which is why a Cocchi Americano and a Dolin taste almost nothing alike and why treating them interchangeably is its own kind of mistake.
The wine part is not incidental. It is the base the whole thing is built on, and wine, once opened, wants to be refrigerated and consumed within a few weeks. The fortification gives vermouth more runway than a table wine, but not unlimited runway — not the open-ended shelf life most bar programs treat it as having.
The objection is cost. A bottle of vermouth refrigerated, consumed within three weeks, and replaced on schedule costs more per drink than one treated as a permanent shelf ingredient. The calculation is accurate and gets the logic backwards. What proper handling actually buys is a drink that tastes like the thing it was supposed to be. The difference between a Martini made with fresh Noilly Prat and one made with the same vermouth after five or six weeks open at room temperature is not subtle. There is no good gin that compensates for it. The tendency when vermouth has oxidized and gone thin is to use less of it, which produces a drier drink but collapses what should be a three-way dialogue between gin, vermouth, and dilution into something with one voice doing all the work.
For dry vermouth, Noilly Prat Original Dry and Dolin Dry are reliable and available almost everywhere. For sweet, Carpano Antica Formula if the Manhattan needs richness; Punt e Mes if it needs more backbone. All of them belong in the refrigerator. The habit that most often gets skipped: replace the bottle when you've worked through roughly two-thirds of it, not when it's empty. Vermouth doesn't fail at a specific moment — it declines gradually, and the last third of an open bottle is already underperforming before most bars notice.
A month or so under refrigeration is the working rule. Mark the date on the label with a piece of tape when you open it. When the tape reads too old, pour out what remains and start fresh. The mechanics are not complicated. The difference between bars that follow something like this and bars that don't shows up in every stirred cocktail on the menu — not as a specific flaw anyone can name, just as the persistent, low-grade sense that something is a little off.