Pour vodka, Cointreau, sugar syrup and grapefruit juice into a large jug. Add 8 stems of lemongrass, crushed but still in one piece. Stir and chill until ready to serve. Fill with ice then serve in martini glasses, with a stick of lemongrass in each.
To make your own sugar syrup put equal quantities of water and caster sugar in a pan and stir over a low heat until the sugar has dissolved. Cool before using.