Vibration monitoring is critical in several disciplines, such as machine and civil engineering, structural health monitoring, biomedical diagnostics, and interactive technology. Mechanical vibrations can cause structural deterioration in engineering systems, or manifest as symptoms in neurodegenerative diseases. As an alternative approach to accelerometers for vibration sensing, carbon-based polymer composites with piezoresistive responses show great potential for detecting and monitoring vibrations. This study investigates the vibration sensing capabilities of polymer composites incorporating carbon black and carbon nanotubes embedded in polyurethane and polylactic acid matrices, and processed via additive manufacturing. Vibratory response tests were conducted on thin rectangular cantilever specimens using a custom-built vibrometer. In a typical free vibration test, a single air pulse is applied to induce oscillation, while the electrical resistance of the specimen is continuously recorded. The oscillatory changes in electrical resistance are correlated with the mechanical vibrations of the specimens, which are also independently measured by optical methods. The natural frequency and vibration amplitude are extracted from the electrical response of the material and validated with independent optical measurements of vibration. The measured natural frequencies are compared to vibratory analytical models. Carbon-based polymer composites demonstrated excellent capabilities to monitor vibratory stimuli and measure natural frequencies through their electrical responses, supporting their great potential for vibration-sensitive applications. These applications range from structural health monitoring to tremor detection in patients with neurodegenerative diseases. Additional advantages are offered by additive manufacturing in terms of fabrication simplicity and design flexibility.