Use "MKLINK /D link target"command to link a directory to another location. In a command prompt opened as administrator. This is native to Windows stuff.
Use it for almost anything.
Two changes i made: launcher seems to duplicate and patch a copy of engine (4.7 preview is sitting at 20GB for me) before replacing it. And VaultCache (store content) is just fine on my HDD instead of my small C-drive/SSD.
cd \Program Files\Epic Games\Launcher
(First move PatchStaging and VaultCache to other drive, root of E:\ for me with Windows Explorer)
mklink /d PatchStaging E:\PatchStaging
mklink /d VaultCache E:\VaultCache
Directory of C:\Program Files\Epic Games\Launcher
02/02/2015 07:24 PM <DIR> Backup
12/11/2014 01:19 AM <DIR> Engine
02/12/2015 07:28 PM <SYMLINKD> PatchStaging [E:\PatchStaging]
01/07/2015 02:31 AM <SYMLINKD> VaultCache [E:\VaultCache]
Make sure the /d is there and it says SYMLINKD.
This works fine with 4.6, 4.7 engine folders. This works fine with Unreal Projects folder in your Documents folder.
edit: updated Launcher Troubleshooting guide: Tips and Tricks