Correct. RenderTargetBitmap is new to Windows 8.1. There is no direct way to render the visual tree in Windows 8. I believe there is a third party toolkit (either WriteableBitmapEx or WinRTToolkit) which will parse the visual tree and
render each element on its own, but I don't know how complete that implementation is. You might give it a try to see if it meets your needs.
Or upgrade to Windows 8.1.