1. A method for performing reference-based overlay metrology using a virtual measurement image, comprising: acquiring a plurality of reference measurement images from a plurality of reference overlay target sites of a wafer via a reference image sampling process, wherein the reference image sampling process includes acquiring one or more images at each of the plurality of reference overlay target sites of the wafer; generating an aggregated overlay target reference image by combining the plurality of reference measurement images acquired from the plurality of reference overlay target sites of the wafer of the reference image sampling process; acquiring one or more measurement images from an overlay target site of the wafer via a measurement image sampling process; and measuring a virtual overlay of the one or more measurement images by comparing the one or more measurement images acquired from the overlay target site of the wafer to the generated aggregated overlay target reference image.