When Visual Studio's (2010, 2012, 2013) help preference is set to Launch in Browser F1 requests will automatically ignore any locally installed help and will attempt to resolve to page hosted at msdn.microsoft.com.

You can workaround this issue and set Visual Studio to check for locally installed help (3rd party content only) first by adding the following DWORD value in the registry and set its value to 1:

32bit

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Help\v2.0\Catalogs\VisualStudio11\VendorContent

64bit

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Help\v2.0\Catalogs\VisualStudio11\VendorContent

More information on this can be found in an MSDN article here: http://msdn.microsoft.com/en-us/library/dd627473.aspx