The penultimate loop iteration could pick any vl such that:
 vlenb/4 < vl <= vlenb/2
Thus if the total length is not a multiple of vlenb/2, the vfadd.vf
on the penultimate iteration would yield corrupt values for the last
iteration.

To avoid this, force vl = vlen/2 until the last iteration. Unfortunately
this latent bug is not reproducible with either hardware or QEMU as of now.