This article presents a novel composite vector bending sensing system capable of switching freely between wavelength and intensity modulation. The fundamental sensing unit of the system depends on an offset rotation twisted Mach–Zehnder interferometer (TW-MZI) with unique refractive index (RI) distribution characteristics proven through experiments. The sensing unit delivers remarkable performance in intensity modulation, reaching a maximum of 18.74 ± 0.01 dB/ $\text{m}^{-1}$ . The sensing unit features a nonuniform geometric distribution and utilizes polarization-maintaining optical fibers as its structural material, enabling it to detect the correlation of polarized light and recognize the bending direction. With these advantages, a quasi-parallel distributed curvature sensing system is constructed, integrating a Sagnac interferometer (SI) as the sensing reference arm combined with the TW-MZI sensing unit. Adjusting the interference length and optical power can obtain a first-order harmonic vernier spectrum. Additionally, the envelope differencing technique enhances the interference fringe contrast, reducing the reliance on the monitoring wavelength domain. This combination highlights the composite system’s wavelength modulation-based vector curvature sensing capability, achieving a maximum value of 15.59 ± 0.01 nm/ $\text{m}^{-1}$ . Moreover, this system exhibits characteristics such as low-temperature crosstalk, high stability, and rapid responsiveness, making it well suited for applications in medical device diagnostics, oil and gas pipeline inspection, and structural safety monitoring within the construction industry.