Windows XP:
C:\Documents and Settings\All Users\Application Data\ASGVIS\VfS\vfs_stderr.txt
Windows Vista, Windows 7:
C:\ProgramData\ASGVIS\VfS\vfs_stderr.txt
On Mac OS X:
/Applications/ASGVIS/VfS/vfs_stderr.txt
The error log is reset every time you open the host application and isn't re-saved unless the host application closes successfully. Wherever possible, close the application (eg. Sketchup) before it crashes in order to record the greatest amount of error information.
The V-Ray error log is particularly useful for identifying problem textures/materials/bitmaps as well as issues with paths of loaded-in components.