The behavior of vector singularities in partially coherent inhomogeneously polarized optical beams, such as U contours with zero degree of polarization, is investigated by computer simulation for the most important cases of incoherent mixing of orthogonally polarized components (speckle fields and plane waves). Vector singularities in partially coherent combined beams are considered within the notion of the complex degree of polarization, with representation in the Stokes space. The dependences of U singularities on the intensity ratio obtained by simulation are compared with obtained early qualitative experimental results.