Tip the sugar and 125ml/4fl oz water into a saucepan. Scrape in the vanilla seeds and put in the pod as well, then heat gently until the sugar has dissolved. Bring to the boil and boil for 6-8 minutes to make a sugar syrup. Leave to cool and remove the vanilla pod.
Halve the melons, scoop out and discard the seeds, then scoop the flesh onto a board and chop to a slush with a sharp knife (or you can do this in a food processor). Mix the sugar syrup and melon slush together with the lemon juice in a large shallow baking dish. Freeze uncovered for about 1 hour until crystals form around the edges. With a fork, stir the edges into the centre and mash well, then return to the freezer until crystals form again. Stir and refreeze as often as you can (say every half an hour or so) until the granita is frozen all over – this should take about 3 hours. Fork it up roughly when you serve it.