If the vendor is patching kernel they probably must release their patches and proprietary drivers as GPL requires (regarding drivers they could avoid this by moving important parts into userspace).
There's two things that happen: one is adding more hacks in the kernel that while required to be released under the GPL (and sometimes are, sometimes aren't!) are never going to reach the mainline kernel in a thousand years because they are way too hacky.
The other is creating userspace blobs that get passed void* from the kernel containing (kernel) internal data structures. The problem then comes about when the kernel changes those data structures.
The other is creating userspace blobs that get passed void* from the kernel containing (kernel) internal data structures. The problem then comes about when the kernel changes those data structures.