Combine vinegar, salt and sugar in a non-reactive saucepan over low heat; stir until salt and sugar are dissolved, about 5 minutes. Remove from heat and stir in water. Let cool to room temperature, about 15 minutes.
Place sugar-snap peas, tarragon, garlic, lemon zest, dill seed and chilli flakes into a large wide mouth jar. Pour vinegar mixture into the jar.
Cover jar and refrigerate until peas are lightly pickled, at least 1 day and up to 2 weeks.
Be sure to trim the ends off the pea pods to allow a full immersion of the brine.