Calculation of QD using VASP is not that much different from bulk calculations; you just need to ensure that sufficient vacuum is applied in all 3 spatial dimensions. My experience is that 10-15 Angstrom is sufficient, but this needs to be tested for your system and property of interest.
The trends in many optical and electronic properties (such as absorption energy) of QD are reliable using conventional DFT with semi-local functionals such as PBE. It is well known that the first exciton peak decreases in energy as the size of the QD increases, due to the loss of quantum confinment effect.
Another important thing to note is that ligands also play a very important role at controlling the optical properties. They are different from "bare" QD. In general the surface needs to be fully passivated with ligands while ensuring overall stoichiometry.