Low-pressure plasma techniques are versatile tools for surface engineering of polymeric materials. A wide range of physical and chemical effects can be achieved in the near-surface region while volume characteristics are preserved. However, some applications are based on modification effects in the outermost molecular layer only, while others rely on a particular depth profile that can be introduced by a plasma treatment. To exploit the latter case to its full capacity, diagnostic techniques capable of depth profiling on a sub-micron scale are required. Along this line, the current study aims at 3D characterization of plasma-treated polydimethylsiloxane. Upon plasma exposure, the material undergoes a gradual conversion from the polymeric structure toward a SiO₂-like structure. The range of this process is limited on a sub-micron scale and depends on the type of plasma and the applied process parameters. Since the above-mentioned conversion involves characteristic changes of optical properties, spectroscopic ellipsometry is a promising diagnostic tool. To fully cover the spectral range with the most pronounced modification effects, vacuum ultraviolet spectroscopic ellipsometry was employed for data acquisition. A two-segment graded ellipsometric model was considered to ensure an adequate representation of the actual depth profile. The approach was applied to different cases of low-pressure O₂, H₂, N₂, and CO₂ plasma treatments. We were able to correlate distinct process parameters of a low-pressure plasma treatment to the depth-dependent material properties in the near-surface region. This paves the way for applications, including, but not limited to, large area microstructure formation on polymer surfaces by plasma-induced wrinkling.