
Grapefruit vodka is a flavored vodka — a neutral base with a predominant grapefruit flavor.
Plain vodka with fresh grapefruit juice is the nearest swap: the flavored bottle carries the fruit’s scent and none of its bitterness, and the juice brings both. Count on the juice lengthening the drink, so cut whatever else was going in to fill the glass.
Grapefruit liqueur
Flavored vodka has to stay at spirit strength — 37.5% ABV minimum in the EU, 30% in the US — and a grapefruit liqueur sits well below that, with sugar on top. Swap it in and the drink comes out sweeter and several degrees weaker.
Grapefruit is the most delicate of the flavored vodkas to keep, because the note that identifies it is a thiol present in vanishingly small quantities and easily lost. An opened bottle a year old will still taste of citrus, just not particularly of grapefruit.