Why vista only? Is it 64 bit only? Otherwise for TUI-only this should go back to at least NT4. Or is it using anything clever that makes older versions barf?
Edit: Ah, maybe it's bypassing ntdll and syscalls changed?
Author here. It's Vista+ because Cosmopolitan focuses on x86-64. Cosmopolitan does however support i8086 since these executables boot from BIOS. You can even define a _start16 entrypoint if you want to write real mode apps. No effort has been made to natively target i386, however it's still technically supported since Actually Portable Executables will re-exec themselves under qemu for other architectures.