Stay cables are critical components of cable-stayed bridges, and their dynamic characteristics provide essential information for vibration assessment and structural health monitoring. This study presents a comparative performance assessment of Fiber Bragg Grating (FBG) sensors and PCB accelerometers for field-based dynamic characterization of stay cables. Field measurements were conducted on selected stay cables of My Thuan Bridge under normal operating conditions. Since PCB accelerometers measure acceleration, whereas FBG sensors capture dynamic strain or wavelength-shift responses, the comparison was performed primarily in the frequency domain rather than through direct time-domain amplitude equivalence. The measured responses were processed using signal preprocessing, power spectral density analysis, and covariance-driven stochastic subspace identification. Dominant dynamic frequencies were identified from both sensing systems and compared using frequency agreement, frequency deviation, spectral peak consistency, and practical field applicability. The results show that the common frequencies identified from FBG and PCB measurements are in good agreement, with an overall mean relative difference of 0.94%. The FBG sensors also detected additional candidate frequency components that showed an approximately regular progression consistent with stay-cable vibration, although these components require further validation. These findings indicate that FBG sensing can provide reliable and complementary frequency-domain information for stay-cable dynamic characterization. The study demonstrates the feasibility of using FBG sensors as an alternative or complementary sensing technique to conventional accelerometers for field-based vibration monitoring of cable-stayed bridges.