We give a sharp lower bound to the largest possible Euclidean norm of signed sums of $n$ vectors in the plane. This is achieved by connecting the signed vector sum problem to the isoperimetric problem for the circumradius of polygons. In turn, we apply the sharp bound for the signed vector sum problem to establish a sharp lower bound to the circumradius of the Minkowski sum of $n$ planar symmetric convex bodies. We also determine a tight lower bound to the circumradius of the Minkowski sum of general convex bodies in any dimension independent of their number.