Windows: Users > Your Username > AppData* > Roaming > Chaos > V-Ray for [host application] > vrayneui
Mac: Users > Your Username > .Chaos* > V-Ray for [host application] > vrayneui
* These folders are hidden. For windows see here for instructions and for Mac press Shift + Command + Full stop/period.
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 and could be requested by Chaos if you open a support ticket with them.