It will only work if you have VS 2013 also installed as well as VS 2015.
— Log Output —
[2015.10.17-11.37.52:889]Building UnrealBuildTool in C:/Users/itchr/Desktop/UnrealEngine/...
[2015.10.17-11.37.52:889]Couldn't find ../../VC/bin/x86_amd64/vcvarsx86_amd64.bat; skipping.
[2015.10.17-11.37.52:889]Failed to build UnrealBuildTool.
[2015.10.17-11.37.52:889]CompilerResultsLog: New page: Compilation - Oct 17, 2015, 1:37:52 AM
[2015.10.17-11.37.52:889]LogMainFrame: MainFrame: Module compiling took 0.003 seconds
[2015.10.17-11.37.52:889]Warning: RebindPackages failed because the compiler could not be started.
Are you using the preview from the Launcher or from GitHub? After removing VS2013 from my machines I opened the 4.10 Preview from the Launcher and created a blank C++ project which compiled successfully using the In-Editor button. I also tested adding a new class to the project and editing it to force a hot reload and used the In-Editor compile again. This was tested using Windows 7. Let me know if there is anything I’ve missed along with the OS version you’re using.
Presumably the reason it still works on your machine is that Visual Studio 2013 did not fully uninstall itself, and left some registry keys, and possibly some files behind in the old directory. Perhaps this is why the bug has gone uncaught.
You should be able to repro the bug consistently on a fresh install of Windows, but that is allot of trouble I realize. But you can see for yourself in the code where the issue is. It will never find the VS 2015 install directory as it is using the wrong key.