They document the WinAPI, but how that talks to the kernel is not documented. You can talk to it directly if you want, but there is nothing from Microsoft on how to do that. So if you see those as the true system calls, they are not documented at all.
Well, tiny parts of the NT API (callable from userspace) are documented, but then often with the caveat that they are not stable (in practice, even some undocumented ones can be considered stable if used by enough programs in the wild, especially if they are simple and standalone and have no Win32 equivalent)
The very precise mechanism, though, is extremely unstable. For example virtually every release of Windows (even sometimes SP) changes the syscall numbers. You have to go through the ntdll, which is kind of a more heavyweight version of the Linux VDSO. (The NTDLL approach was invented way before the VDSO, though)
Ntdll is similar to VDSO in the sense that it is loaded into the memory space of every userspace process. Even that I think might have exceptions on the Linux side. Either way, unlike VDSO, Ntdll actually does export functions potentially useful when called from the program. Here is an interesting read. http://undocumented.ntinternals.net/
What do you think the VDSO is used for? It also exports "functions potentially useful when called from the program".
The approach is a little different though; ntdll exports all of the NT API, and you need to go through it to reach the NT API in a somehow more stable way than using syscall numbers. OTOH, the VDSO exports only virtual syscalls that gain (or have gained in the past) from being performed in userspace, and even then corresponding syscalls still exist in the kernel, with both stable numbers and even a stable API.