
Vanilla liqueur is a sweetened spirit flavored with vanilla, usually from cured vanilla pods or vanilla extract.
For a general sweet accent, another rounded dessert liqueur such as an almond one keeps the balance but changes the flavor. When vanilla is the point, use a true vanilla source instead: vanilla extract or syrup let down with neutral spirit in roughly equal volumes, which is what restores the usual 20% ABV, then sweetened to taste with simple syrup.
Vanilla without the alcohol, and noticeably sweeter. The measure holds and the strength has to come from elsewhere. Make the strength up with neutral spirit, roughly on par with the syrup; cut the syrup in the build to match.
Vanilla on a neutral base with no sugar at all, around 37.5% ABV against 20%, and pour half the measure and let the syrup already in the recipe carry the sweetness.