In mode analysis for noncircular, inhomogeneoua and anisotropio dielectric waveguides shift formula methods are widely developed. By these methods the mode parameters and fields of the investigated complex waveguide are calculated by using the known mode parameters and fields of reference waveguide, e.g. circular homogeneous and isotropio waveguide. In order to simplify the calculations of reference waveguide mode parameters, vector successful (VS) approximation is correspondingly used as well. In this paper the accuracy of this approximation is checked and some generalized VS-approximations are proposed, which are quite useful in applicaitons.