/Log (devenv.exe)
Logs all activity to the log file for troubleshooting. This file appears after you have called devenv /log at least once. By default the log file is:
%APPDATA%\Roaming\Microsoft\VisualStudio\<version>\ActivityLog.xml. However, you may specify an alternate path and file name.
where <version> is the Visual Studio version.
Devenv /log <path/name of log file>
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 have invoked with the /log switch. It does not log instances of Visual Studio that you have invoked without the switch.