LibreOffice for Windows uses Mantle. LibreOffice for linux has had OpenCL acceleration for quite a while, as well as OpenGL acceleration.
Windows has the problem that everything is locked in, and that the tools for developers to target OpenCL and OpenGL, which are basic APIs in linux, are different from the tools for targeting DirectX. This means that developers have to invest twice if they want to target Windows and something else, like linux. That is the problem that Mantle solves, it bypasses the DirectX API and offers developers a standardized API that targets the Khronos spec API trackers in different operating environments. Most linux devs are quite experienced with working with the standard Khronos spec API's though, and because it's open source, it's not necessary to redo all the work that has been done before, like for instance, the display server will take care of stuff, existing engines and modules will take care of stuff, and the devs can recycle those without having to develop from scratch like in Windows. The end result between using Mantle to develop for Windows and linux with the same effort, or developing for OpenCL/OpenGL in separate efforts for Windows and linux, is the same, Mantle API just makes it easier to develop because it offers an alternative API, but it just taps into the things that are already there in linux, while it adds some of these things to Windows (like CPU scheduling, etc...), but it can't add all the things to Windows, linux will still have an advantage.
On the other hand, the KMS drivers in linux right now are only compatible up to OpenGL 2.3 or so, whereas the proprietary drivers are compatible with OpenGL 3.3 instructions. So there is still work to be done on the drivers, but those are evolving really rapidly.
In any case, the VCE thing is not available in Windows to the same extent, because Windows doesn't have KMS graphics drivers. Like if Adobe for instance would decide to develop their newest Premiere version with Mantle API, it would benefit the performance in Windows, but not to the same extent as in linux, because in Windows, the Mantle API programmed calls would tap into a userspace subsystem, whereas in linux, they would generate the ordinary OpenGL and OpenCL calls that tap into the kernel itself. The difference in performance should be rather phenomenal just because of that, and so is the difference in features. With less code, you could do more by targeting KMS drivers than you would be able to achieve by targeting userspace drivers, even if you would be using the same API (Mantle) to target those features in your application. Open source software would also not really need Mantle, because even if it were developed initially with the Mantle API, the community would jump on on it and optimize it for the most features and direct tracking by the instructions merged into the KMS driver in the kernel. So Mantle is primarily a Windows-targeted product, but it also serves to unlock extra performance in linux for applications that were developed with Mantle API for Windows.