Up through vSphere 4.1, VAAI is fully supported on the CX4.  Starting in vSphere 5.x, it gets muddied.  VMware will officially say no it is not supported because not all features are supported, but some features will work.  Here is VMware's article on it:  [URL]  Here is a link to Chad's blog that goes into details as to what works and what doesn't.  [URL]
Does the CX4 (FLARE 30) support VAAI?
Does the CX4 support VAAI?