Squeeze the grapefruit juice and pour it into a sandwich bag with the fish sauce. Add the tuna piece. Close the bag, removing the air so that the tuna is completely covered in the juice. Let it rest for 40 minutes, after which the outside of the tuna will be pale and ‘cooked’. Now, carefully pour the grapefruit juice into a bowl, dry the tuna, and set it aside. For the dressing, mix the sesame oil, olive oil, and chilli in the grapefruit juice. Use more chilli to taste and season to taste. Pull off a good handful of coriander and mint from the bunches and set them aside for garnish later. Finely chop the remaining herbs and tap them around the tuna to coat. Wrap in cling film and refrigerate until use. Boil the vermicelli for 1 minute until they are slightly flexible, then drain and let cool. Add a little oil to a hot non-stick pan, add the vermicelli, and let them cook until they are crispy on one side. Flip them over and do the same on the other side – it’s okay if some stick to the pan, simply unstick them and flip them over. Divide the crispy vermicelli among 4 plates. Slice the tuna into approximately 0.5cm/¼ inch thick slices. Place the tuna on the noodles, sprinkle the herbs set aside earlier, sprinkle the scallions, and drizzle the tuna with a few spoons of the dressing. You’ll see the cut sides of the fish slices start to change color and ‘cook’. Serve immediately.