Well in short as far as I see it:
Vulcan has a great design, and jme would benefit from this
Early drivers will most likely suck, so lets just wait a little more till it stabilizes.
Never implement a 1.0 API, just compare opengl 1.0 and 1.4 for example, it is usually better to wait a bit here, as else you have to support all kind of old crap forever. cough Remove OpenGL 1 in jME 3.1? Noooooo! - #25 by prog cough we should have never added fixed pipeline support in the 3.x
GLSL is promised to be compatible for the new language as a first supported intermidiate language.
It needs to be determined what else must be changed for vulcan support,
best case → only an additional rendere is necessray (unlikely)
expected case → some modifications to the core are required, to allow it to support vulcan properly
Especially given mantles fast rise and fast dissaperance lets just wait out while the new next gen api is sorted out.