Sprinkle the gelatine over 3 tbsp cold water in a small, heatproof bowl. Leave to stand for a few minutes until spongy, then place the bowl in a pan of gently simmering water until the gelatine dissolves. Set aside.
Pour the cream into a saucepan and tip in the sugar. Split the vanilla pod in half lengthways, scrape out the black seeds and whisk into the cream. Set the pan over a medium heat and bring the cream to the boil. Take the pan off the heat and stir in the gelatine.
Pour the milk into the hot cream, give it all a good stir and leave to cool for an hour or so. Put the pan in the fridge for 1-2 hours until the cream just begins to thicken, stirring it every now and then to evenly distribute the vanilla seeds. Pour the cream into a 1-litre serving dish, cover and refrigerate for at least 5 hours (overnight is ideal) until completely set. The cream will sit in the fridge for up to two days.
To serve, halve, stone and thinly slice the nectarines, then toss with the lemon juice and peach vodka if using. Scatter the nectarine slices over the surface of the set cream. Serve immediately, decorated with lemon balm or mint leaves.