
Vanilla bitters are a high-strength cocktail bitters flavored mainly with vanilla, dashed in as an aromatic accent.
ABV Strength varies a lot by maker, and no single figure describes the category. It makes no difference to the drink either way — these go in by the dash.
There is no clean substitute. A few drops of vanilla extract carry the aroma but bring sweetness the bitters do not, and none of the bitterness; another aromatic bitters plus a drop of extract lands closer than either alone.
Vanilla bitters go in by the dash — high proof, almost no volume, almost no sweetness. Vanilla syrup is the opposite: sweet, barely alcoholic if at all, and poured in quantity. Swapping one for the other changes the sugar in the glass, not just the aroma.
Vanillin is stable, and this is the bitters that ages best of all of them. The one thing that changes is the sediment: natural vanilla extract throws a fine dark deposit over time, which is harmless and worth a gentle roll of the bottle before use.