In binary aluminosilicate liquids and glasses, heterogeneity on intermediate length scale is a crucial factor for optical fiber performance, determining the lower limit of optical attenuation and Rayleigh scattering, but also clustering and precipitation of optically active dopants, for example, in the fabrication of high-power laser gain media. Here, we consider the low-frequency vibrational modes of such materials for assessing structural heterogeneity on molecular scale. Using low-temperature heat capacity and low-frequency Raman scattering data, we obtain an accurate scaling of the vibrational density of states and the Boson peak. This allows for the extraction of the average dynamic correlation length as a function of alumina content. It was found that this value decreases from about 3.9 nm to 3.3 nm when mildly increasing the alumina content from zero (vitreous silica) to 7 mol%. At the same time, the average inter-particle distance increases slightly due to the presence of oxygen tricluster species. In accordance with Loewensteinian dynamics, this proves that mild alumina doping increases structural homogeneity on molecular scale. On a broader perspective, these findings shine new light at the structural origin of the excess in the vibrational density of states which manifests in the non-Debye behavior of non-crystalline materials.