In the paper, a comprehensive fully self-consistent three-dimensional simulation of an operation of the GaAs-based oxide-confined long-wavelength 1.3-μm quantum-dot (QD)(InGa)As/GaAs vertical-cavity surface-emitting diode lasers is demonstrated. The model has been intentionally prepared for the PC-class microcomputers to enable its easy application in designing optimal structures of the above devices with desired performance characteristics. An impact of some structure parameters on QD VCSEL room-temperature (RT) continuous-wave (CW) lasing thresholds is discussed. A stable RT CW operation on a single fundamental mode has been found to be possible in modern QD VCSELs with active regions containing more uniform and more dense QDs in stacks of QD layers. The desired single fundamental-transverse-mode operation is possible for smaller active regions of diameters not exceeding 8 μm. In the case of larger active regions, on the other hand, higher-order transverse modes of an increasing order are excited first because of increasingly more non-uniform optical-gain distributions.