A modified solid fiber-optic integrated Mach-Zehnder interferometer (IMZI) assisted with a thin piezoelectric sheet (TPS, ~ 130um thickness) and phase generated carrier (PGC) demodulation is proposed and investigated theoretically and experimentally. The IMZI's two arms are respectively molded with epoxy resin (EP) and silicone rubber (SI). These two materials are with distinctly different elastic modulus and Poisson ratios, which guarantees the high sensitivity of the IMZI. As for PGC, different modulation frequencies and amplitudes are generated and the corresponding modulation depths are calculated to obtain the optimal modulating parameters. Furthermore, a pair of complementary photodetectors are used to remove unwanted DC component and only the first harmonic is utilized to recover the phase, which can effectively avoid signal fading. In the experiments, where the IMZI is put under the mattress, activity monitoring is addressed and subjects are recruited to investigate the vital signs monitoring performance. The consistency check analysis results show the calculated heart rate variability (HRV) and breath rate variability (BRV) results are highly correlated with reference results, and the Pearson's r reaches 0.99. Moreover, reliability, post-exercise, and Valsalva Maneuver test are further conducted to verify the repeatability and dynamic monitoring capability. In conclusion, the proposed system is unobtrusive, cost-effective, robust, and convenient, which has great potential in future homecare and hospitalization. IEEE