Possibly because vdev is a clean break, while eudev is still mostly about udev stripped from systemd.
And that stripping will be more complicated moving forward, as i recall a recent systemd release moved various bits from udev to a new systemd lib. Leaving the udev interfaces as stubs to be removed at some undetermined future date.
Have you a notion as to why they're using vdev rather than eudev? What appears to be the vdev introductory blog post makes no mention of eudev.