Make a syrup with the water and 1 1/2 cups of the sugar by bringing them to the boil in a saucepan and boiling for 5 minutes. Take the pan off the heat, add the lemongrass and let it infuse for about half an hour.
Strain the syrup into a measuring cup, keeping the saucepan with the lemongrass to 1 side. Take out about 1/2 to 3/4 cup of the syrup and put it into a pan with the raspberries. Bring it to a rolling boil and let it thicken slightly, mashing the fruit to make a jam-like consistency. Let it cool a little and then dunk the ladyfingers in the raspberry mixture and arrange them in the bottom of your bowl. Add the vodka and about 1/2 cup of the lemongrass syrup, depending on how much your ladyfingers absorb, and reserve the rest.
Meanwhile, to make the custard, heat the light cream in the syrup pan with the lemongrass until it is nearly boiling, take it off the heat and let it infuse for about 15 minutes. Strain into a bowl. Whisk the yolks and the remaining 1/4 cup of sugar together and pour into the cream in the bowl. Then whisk again and put the custard back on the heat in the cleaned-out pan. Stir until the custard thickens and then pour it over the trifle sponges. Let it cool.
Whip the heavy cream until thick but not stiff, and cover the custard layer. Use about 1 cup of the remaining sugar syrup to make a caramel by heating it in a saucepan until it turns a golden brown. Drizzle the caramelized sugar syrup over the layer of cream to decorate.