http://blogs.msdn.com/b/vcblog/archive/2015/04/29/c-11-14-17...
"Q. What about the C99 Core Language, or the C11 Core Language and Standard Library?
A. Our top priority is C++ conformance. We've implemented the C99 Standard Library because C++11 incorporated it by reference. C++ (up to the current Working Paper) hasn't incorporated the C99 Core Language in its entirety (only a handful of features, listed above), nor is it ever likely to. It may incorporate the C11 Standard Library at some point in the future, but that hasn't happened yet.
In VS 2013, the compiler team implemented some C99 Core Language features in C mode (e.g. designated initializers, see MSDN for the full list), in order to support some popular libraries. "
There was a C9 interview about it, need to search for it.
https://channel9.msdn.com/Events/Visual-Studio/Launch-2013/V...
Starting at 00:18:40, note the "not an intent to conform to C99/C11".
FFMpeg was one of the customers
If I recall correctly it was at a Visual Studio event last year, on those floor interviews they publish with the teams on C9.
https://helpx.adobe.com/creative-suite/kb/microsoft-visual-c...
There's probably quite a few internal-only users of Visual C++ too.
EDIT: Well, they're a high-profile user of Visual C++, I have no idea if they require C99 support but they ship a Mac version so it's possible.
Here's an older article - http://blogs.msdn.com/b/vcblog/archive/2013/07/19/c99-librar...
(I've seen more recent ones, but I can't find an example to hand)