Late reply here, but in a recent post Juan said that he would begin working on the Vulkan port in May.

https://godotengine.org/article/godot-32-will-get-new-audio-... (at the bottom)

In the post I previously linked he said he expected the port to take a "couple of months", so we could expect it in any release made in the 3rd or 4th quarter this year, maybe as early as July?

Notably, the article being from February 2018 tells a lot.