Spin-orbit interactions are inherent in many basic optical processes in anisotropic and inhomogeneous materials, under tight focusing or strong scattering, and have attracted enormous attention and research efforts. Since the spin-orbit interactions depend on the materials where they occur, the study of the effects of materials on the spin-orbit interactions could play an important role in understanding and utilizing many novel optical phenomena. Here, we investigate the effect of negative-index material on the spin-orbit interactions in a plasmonic lens structure in the form of a circular slot in silver film. Numerical simulations are employed to study the influence of the negative-index material on the plasmonic vortex formation and the plasmonic focusing in the structure under circularly polarized excitations bearing different orbital angular momentum. We reveal that the presence of negative-index material leaves the plasmonic vortex field distribution and the corresponding topological charge unaltered during the spin-to-orbital angular momentum conversion, whereas reverses the rotation direction of in-plane energy flux of the plasmonic vortex and shifts the surface plasmon polariton focus position to the opposite direction compared to the case without negative-index material. This work will help further the understanding of the regulation of optical spin-orbital interactions by material properties and design optical devices with novel functions.