Raman spectra of a (DRADP-25) mixed crystal have been investigated in the 8 - 300 K temperature range for all fundamental polarization geometries. As in the case of DRADP-50, the spectra can be interpreted in terms of vibrational modes of principal structural units. Analysis of the temperature dependence of peak positions and halfwidths has shown two characteristic temperature regions (200 - 220 K and 110 - 120 K) where incipient cluster formation and freezing of individual structural units takes place in this compound on cooling through the dipolar glass transition. In contrast to those for DRADP-50, the Raman spectra of DRADP-25 reveal additionally the onset of incipient ferroelectric clustering and the existence of a low-temperature mixed ferroelectric - glass phase.