  We have developed a new potential energy surface for spin-polarized K($^2$S)
+ K$_{2}(^3\Sigma^+_u)$ collisions and carried out quantum dynamical
calculations of vibrational quenching at low and ultralow collision energies
for both bosons $^{39}$K and $^{41}$K and fermions $^{40}$K. At collision
energies above about 0.1 mK the quenching rates are well described by a
classical Langevin model, but at lower energies a fully quantal treatment is
essential. We find that for the low initial vibrational state considered here
($v=1$), the ultracold quenching rates are {\it not} substantially suppressed
for fermionic atoms. For both bosons and fermions, vibrational quenching is
much faster than elastic scattering in the ultralow-temperature regime. This
contrasts with the situation found experimentally for molecules formed via
Feshbach resonances in very high vibrational states.
