/Log (devenv.exe)
Logs all activity to the log file for troubleshooting. This file appears after you've called devenv /log at least once. By default, the log file is:
%APPDATA%\Microsoft\VisualStudio\Version\ActivityLog.xml
where Version is the Visual Studio version. However, you may specify a different path and file name.
Devenv /log Path\NameOfLogFile
Remarks
This switch must appear at the end of the command line, after all other switches.
The log is written for all instances of Visual Studio that you've invoked with the /log switch. It doesn't log instances of Visual Studio that you've invoked without the switch.