A semi-analytical method to examine the influences of the axial variations in the tube vibration amplitude and flow velocity on the critical flow velocity is investigated. We illustrate that neglecting the axial variation in the tube vibration amplitude can result in an overestimation of the critical flow velocity (nonconservative estimate) when the flow velocity is nonuniform. A condition under which such overestimation arises is derived by the transformation of the eigenvalue problem that is made to take into account the axial variations in the tube vibration amplitude and flow velocity. This condition is the existence of a positive correlation between the deviations of two functions: one representing the axial variation in the flow velocity and the other square of the function representing the nonuniformity of the tube vibration amplitude. The case with marked partial admission is investigated through physical consideration for this flow-induced vibration problem. We also study cases where the difference between tube eigenfrequencies in the flow and transverse directions results in a transition in the instability direction, from the transverse direction to that of flow.