On Windows, the vdenoise.exe is located in C:\Program Files\Chaos Group\V-Ray\Mayannnn Maya20nn for x64\maya_vray\bin where nnnn20nnis the Maya version. You can also start it from Start Menu > V-Ray for Maya nnnn 20nn > Denoiser tool.
On Linux, the vdenoise executable is located in /usr/ChaosGroup/V-Ray/MayannnnMaya20nn-x64/vray/binwhere nnnn where 20nn is the Maya version.
On macOS, the vdenoise executable is located in /Applications/ChaosGroup/V-Ray/MayannnnMaya20nn/vray/VRay.app/Contents/MacOS where nnnn20nnis the Maya version.
