Before revision: For an unweighted graph on kterminals , Kratsch and Wahlstr\"om constructed a vertex sparsifier  with O(k^3) vertices via the theory of representative families on matroids.
After revision: For an unweighted graph on kterminals , Kratsch and Wahlstr\"om constructed a vertex sparsifier H with O(k^3) vertices via the theory of representative families on matroids.