Old question but here is the approach we have used ever since Visual Studio 2005 with success. I just tested it using Visual Studio 2012 Update 4 as well (since we are finally updating our software from 2010 to 2012).
Since the Visual C++ Redistributable packages register their uninstaller with windows (so it shows up in the Control Panel "Programs and Features" list), we simply check for the Display Name of the uninstaller key in the registry.
Here is the relevant NSIS code:
ReadRegStr $0 HKLM "SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{33d1fd90-4274-48a1-9bc1-97e33d9c2d6f}\""DisplayName"StrCmp $0 "Microsoft Visual C++ 2012 Redistributable (x86) - 11.0.61030" vs2012redistInstalledDetailPrint "Microsoft Visual C++ 2012 Update 4 Redistributable not found!"DetailPrint "Downloading from www.mywebsite.com"; insert applicable download code hereExecWait '"<downloaded redist exe>" /promptrestart /passive'vs2012redistInstalled:Note that since our installer is a 32bit exe, windows handles determining if the registry key is actually in the virtualized Wow6432Node instead of the above location so the above code works on both 64bit and 32bit windows installs without having to check both keys explicitly.
Also note that to update the above code to a different version of the VC++ Redist, simply change the GUID in the registry key path and the display name to whatever you need.
While this may not be the recommended method, It has worked on 10,000+ machines over the past 10 years running every flavor of windows from XP/XP64 Through Windows 10 using redists for 2005, 2010, 2010sp1, and now 2012u4.