Combine the mangoes and tomatoes in a large mixing bowl. Scrape in the seeds of the split vanilla pod and add the pod as well (if using essence, add later).
Cover with the sugar and cling wrap the bowl; leave to macerate overnight in the refrigerator. Place the mixture in a medium-sized saucepan (add vanilla essence if using) and cook over low heat for about 1 hour, or until thick, stirring occasionally so that it will not stick to the bottom of the pan.
If there are still large pieces of fruit, use a spatula to mash them up.