Combine Vegeta stock powder with 1 cup (250ml) boiling water. Stir to combine.
Add half the oil to large frying pan over medium-high heat. Add garlic, chilli and fennel to the pan and cook until the garlic is lightly golden. Add the vermouth and flambé before adding the chopped tomatoes and stock. Bring to a boil then reduce heat and simmer for 7 minutes. Remove from heat and blend until smooth. Return to pan and warm over a low heat.
Cook pasta in boiling salted water until al dente, about 6-8 minutes. Drain and return to the warm saucepan with a drizzle of olive oil.
While the pasta is cooking, heat remaining 2 tablespoons olive oil in a frying pan on a high heat and pan-fry the prawns for about a minute or two on each side or until just cooked through.