  To learn more about the properties of the Vela Supercluster (VSCL) located
behind the Milky Way at $cz\sim 18000$~km~s$^{-1}$, we determined the
$K_s$-band Luminosity Function (LF) of VC04, the richest known galaxy cluster
in the VSCL, and two other VSCL clusters (VC02 and VC08). The galaxy sample is
based on NIR observations which are complete to an extinction-corrected
absolute magnitude of $M_{Ks}^o<-21.5$ mag ($\sim 2.5$ mag below $M_K^*$),
within the clustercentric radius of $r_c<1.5~$Mpc ($\sim 70 \%$ of the Abell
radius). For VC04 we obtained 90 new spectroscopic redshifts of galaxies
observed with the 11-m Southern African Large Telescope. We found the Schechter
parameters of the VC04 LF to be $M^*=-24.41\pm 0.44$ mag, $\alpha=-1.10\pm
0.20$ and $\phi^*=8.84\pm 0.20$. Both the redshift data and the LF confirm VC04
to be a rich, not yet fully relaxed cluster. We independently determined the LF
of VC04 on membership defined by the Red-Sequence method and demonstrated that
this method can be used in the absence of high spectroscopic coverage over a
cluster. This allowed us to determine the LFs of VC02 and VC08. We also derived
the LFs of the Coma, Norma and Virgo clusters to similar depth and extent as
the VSCL clusters. We found that the Schechter parameters of VC04 are within
$1\sigma$ uncertainties of these local clusters, as well as VC02 and VC08. We
do not find significant differences between the LFs in the different cluster
environments probed in this work down to $M_K^*+2.5$ mag.
