In a large pot, heat butter or oil over medium heat. Add leeks and sweat, stirring often, until soft, about 6 minutes. Add potato, vermouth and chicken stock and bring up to a simmer. Cook until potatoes are tender, about 15 minutes. Add peas and sprigs of herb (tie them with a string to keep them together). Bring just up to a simmer, remove herbs and purée soup until smooth. Strain for a smoother soup (optional) then season to taste and serve immediately garnished with sour cream or crème fraiche.