Comprehensive three-dimensional ray tracing studies suggest that local large-scale ionospheric density perturbations caused by the HF heater at Arecibo significantly affect the amount of power from upgoing 28.5 kHz VLF transmissions that couples into plasmaspheric ducts on nearby magnetic field lines. Large density depletions can perturb the ray paths of upgoing VLF to enhance or reduce the total power in a local duct. Under certain conditions, ducted power may be enhanced up to 13 dB or attenuated up to 18 dB. The dependence of this effect on the location and cross-L drift of plasmaspheric ducts and the depth of the heater-induced depletion may explain the difficulty in obtaining consistent experimental observations. Nongeometrical optics effects from the heating and resultant structures on VLF propagation may also affect the ducted power distribution but are outside the scope of ray tracing analysis.