Probing the properties of the interfacial layer between the polymer matrix and nanoparticles in polymer nanocomposites (PNCs) remains a challenging task. Here, we apply three methods-a single Havriliak-Negami (HN) function fit, a two HN functions fit, and the heterogeneous model analysis (HMA)-to analyze the dielectric spectra of model poly(vinyl acetate)/SiO2 nanocomposites for the thickness and the average slowing down in dynamics of the interfacial layer. We find the HMA presents the most accurate analysis on both the thickness and dynamics of the interfacial layer, in comparison to the other two methods that have been actively applied in the field. In addition, the dielectric spectra at low temperatures reveal unexpectedly nonmonotonous changes in the secondary relaxation of the polymer with nanoparticle loadings. These results clearly demonstrate that dielectric spectroscopy is an easy and robust method to study a wide range of dynamic properties of the interfacial layer in PNCs.