Here's my experience with AMD Ryzen CPUs on Vista: they work great!
There are no random BSOD or services not starting like Haswell or above. USB 3.x drivers for Windows 7 work just fine on Vista, but you can't slipstream them into the installation, because they are not signed for NT 6.0. You'll need to have a PS/2 keyboard at hand (you only really need the keyboard). A few drivers you should not install, because they do not work:
AMD PCI
AMD GPIO (this one will make your system unbootable!)
Everything else works just fine. I've used a MSI B350 PC Mate motherboard with 16GB of DDR4 RAM. Had to enable legacy USB, enable CSM and disable Secure Boot. After that, I was able to boot Vista in UEFI.
Hope this (really late) post helps, since you've asked ages ago @WinClient5270