Photoemission electron microscopy (PEEM) is used to study Ag surface diffusion on vicinal Si surfaces. The diffusion field is represented by Iso-Coverage Zones around Ag islands during desorption. By analyzing the shape and radius of the Iso-Coverage Zone we can determine diffusion parameters. For anisotropic diffusion the zone has an elliptical shape and the aspect ratio gives a measure for the anisotropy. Using this technique, we study the degree of anisotropy of Ag diffusion on vicinal Si(001) and Si(111). With increasing miscut angles, starting from Si(001) as well as from Si(111), we find a gradually increasing anisotropy, caused by the higher step density. On higher index surfaces, like Si(119), Si(115) and Si(113), we find isotropic diffusion for surfaces with comparable dimer and (double) step structure as on Si(001)-4°, where diffusion is strongly anisotropic.