Unable to open existing projects in VS2015

Hello,

When attempting to open an existing, fully functional Qt project created with Qt Creator in Visual Studio 2015, the following error is written to the console and the project does not open. 

 --- (Import): Generating new project of QQmlMdiTest.pro file
--- (qmake) : Using: C:\QT\5.9.1\MSVC2015_64\bin\qmake
--- (qmake) : Working Directory: C:\Users\Dev\Documents\Qt\QQmlMdiTest
--- (qmake) : Arguments: -tp vc "QQmlMdiTest.pro" -o "QQmlMdiTest.vcxproj" QMAKE_INCDIR_QT=$(QTDIR)\include QMAKE_LIBDIR=$(QTDIR)\lib QMAKE_MOC=$(QTDIR)\bin\moc.exe QMAKE_QMAKE=$(QTDIR)\bin\qmake.exe
[1] - Project ERROR: Cannot run compiler 'cl'. Maybe you forgot to setup the environment? 
--- (Import): Error(s): 1
--- (qmake) : Exit Code: 3

  // Additional Information

• Qt Creator works perfectly.
• VS can create and run NEW Qt projects created in VS from the New Project Wizard without issue.
• It is only the opening of existing projects that throws this error.
• This was not an issue in 5.8
• I have both reinstalled Qt / VS 2015 on my development machine AND set up a fresh VM. Both throw this error.
• This issue does not seem to be project specific. Even creating a simple console application with QtCreator then attempting to open with VS fails.

 // How to reproduce:

• Install MS Visual Studio
• Install latest Qt 5.9.1 from website.
• Install Qt VS addin
• Open VS, Select the Qt Menu > Open .pro

If there are new steps required to open existing projects in VS (and it doesn't "just work" anymore) please let me know what they are.

Thanks for your time!