Due to the potential applications in optical storage, optical logic gates and all-optical signal shaping, the polarization switching (PS) and bistability (PB) of vertical-cavity surface-emitting lasers (VCSELs) under external disturbance have attracted much attention. In this work, based on the spin-flip model, the characteristics of PS and PB in a variable-polarization optical injection 1550 nm VCSEL (VPOI-1550 nm-VCSEL) are investigated numerically. In this scheme, the output from a master tunable laser passes through a rotating polarizer, an optical isolator (ISO), and a neutral density filter, then is injected into a 1550 nm-VCSEL. The polarization angle and power of the injection light are controlled by the RP and ISO, respectively. The results show that for a free-running 1550 nm-VCSEL, the parallel polarization-mode (Y polarization-mode) is lasing while the orthogonal polarization-mode (X polarization-mode) is suppressed in the 1550 nm-VCSEL. After introducing a variable-polarization optical injection, for a given frequency detuning Δν (defined as the frequency difference between the injection light and the X polarization mode), type I PS occurs during continuously increasing the injection power and type II PS occurs in the inverse process if the polarization angle θp of the injection light (defined as the angle difference between the polarization direction of injection light and Y polarization mode of the 1550 nm-VCSEL) is large enough. Moreover, the injection power required for generating type I PS is different from that for generating type II PS, namely PB is observed. When Δν is fixed, with the increase of θp, the injection power for the occurrences of the two types PS and the width of PB decrease. For a larger value of |Δν|, the injection power for the occurrence of type I PS is higher meanwhile the width of the PB is larger than that for a relatively small value of |Δν|. On the other hand, for a given injection power, type I PS, type II PS, and corresponding PB can also be observed in the 1550 nm-VCSEL through continuously increasing and reducing θp within the range from 0° to 90° under an appropriate Δν. For a relatively small |Δν|, the value of θp required for the occurrence of type I is similar to that for type II PS, which results in the very narrow width of the PB. Contrastively, for a relatively large |Δν|, the values of θp required for the occurrences of the two types PS and the width of PB severely fluctuate with the variation of Δν. Therefore, for the fixed parameters of the 1550 nm-VCSEL, through adjusting the power and polarization angle of the injection light, the performances of the PS and PB can be optimized. It is expected that this work can provide an effective guidance for optimizing the VCSEL-based bistable devices.