The evolution during growth of the vibrational properties in several plasma deposited a-SiO2 thin films is studied. We mainly compare dense a-SiO2 deposited either at low pressure in an integrated distributed electron cyclotron resonance (IDECR) reactor or at higher pressure in a dual mode reactor. The latter system also allows one to vary the a-SiO2 film density. Growth is studied by in situ infrared ellipsometry on stainless steel and c-Si substrates in the 1000-1400 cm(-1) region. This region corresponds to the Si-O-Si asymmetric stretching mode which is split into transverse optical (TO) and longitudinal optical modes. Vibrational modes are revealed by inflection points in the ellipsometric angle Delta. LO modes are predominantly observed with metallic substrates, whereas on c-Si TO and LO modes are simultaneously observed. Vibrational properties of the IDECR films are found to be very close to the reference thermal silica over the whole range of thicknesses (up to more than 1000 nm), whereas the dual mode plasma deposited oxides have their vibrational modes shifted towards small frequencies and are broadened. This difference is mainly found in films with a higher H concentration which were deposited at high pressure (>9%) rather than in the IDECR film (<2%). The study of the Delta inflection point frequencies versus film thickness also shows differences in the interface formation. Contrary to the IDECR case, the vibrational properties of the dual mode plasma deposited oxides are stable only at large thicknesses. Vibrational properties are then compatible with two Lorentz oscillators : a strong one near 1050 cm(-1) (with a strength F = 0.65 and width 50 cm(-1)) and a weaker one near 1175 cm(-1). In contrast, at small thicknesses, upward shifts in the LO frequency are observed. An interpretation in terms of disorder induced mode coupling at the interface between the two vibrations is proposed. We discuss the possible role of nucleation processes, due to high pressure, in correlation with the observed influence of film morphology and the nature of the substrate.