All implementations (phys2d, jbox2d, slick2d, libgdx, lwjgl-utils, etc) seems to be very similar and to have similar stuff. If all of them work together, we could have a common and better math library.
Vecmath seems interesting to be modified for the greater good but only jbullet is using it, also I dunno about its license.
Vecmath API (there is BSD or so licensed reimplementation btw) is closest to best candidate for two reasons: it is somewhat official (but quite weak to be honest) and technically probably the best you can achieve in Java. And we've tried, me and other devs to come with a better API.
I don't think such unification will happen as everyone has their opinions and it's easier and more coherent to port everything including vector/matrix/etc. routines in ported libraries.
However, in an unlikely event if there would be some evolved vecmath API (basically just added stuff to vecmath api and in unofficial package) and have support in other java gamedev libraries, I would most likely adapt JBullet to it.