The method of coherence observation by interference noise (COIN) [Kinrot , Phys. Rev. Lett. 75, 3822 (1995)] has been shown to be a useful tool for measurements of wave packet motion at the quantum-classical border. We present the first systematic interferometric study of fractional vibrational revivals in the B state of thermal iodine (I-2) vapor. Experimental COIN interferograms ranging from 200 fs to 40 ps are presented for various excitation wavelengths. The complex temporal structure of the observed fluorescence includes rapid initial damping in the short-time regime and the appearance of quarter- and half-revivals on the quantum-mechanical long-time scale. These features arise from a delicate balance between rotational and vibrational molecular coherences. The clear observation of the wave packets on the long time scale is possible due to the long-time stability of the COIN interferometer. Lowest-order perturbative solutions nicely recover the experimental results, and closed-form analytical expressions based upon the factorization approach and the Poisson summation give insights into the nature of dephasing and rephasing of vibrational wave packets subject to rotational inhomogeneous broadening.