It’s part of the MSVC x64 ABI, not a part of the kernel ABI. The kernel and user mode (Win32) APIs all use C linkage so vtables are not relevant and the ABI is a convention of the compiler not the OS. Practically speaking though if your software runs on Windows it will probably use the MSVC ABI.
Yes, you either follow Itanium ABI (non Win) or MSVC ABI. Technically nothing stops _your_ compiler from implementing totally own conventions but it will only be compatible with itself
I'm not a Windows-knower, are vtable layouts part of the user<->kernel ABI?