All of the VLA parameters are, at temporarily, out of use. The old VLA system has been turned off and the EVLA is not yet doing VLBI. The control scheme may change with the EVLA.
VLARFANT can be used to specify the reference antenna for the VLA. This will be the antenna used as the source of data in single dish VLBI. For the phased array, it will be the reference antenna for the calibration solution for phasing. Note that, in phased array mode, the phase center is always at the normal VLA phase center. In single dish mode, if there are multiple antennas available to the VLA correlator, they will be correlated with the phase center at the reference antenna. This is needed so for the old VLA system so that the fringe rotators do not affect the single dish VLBI data.
Normally the reference antenna is number 10, which is the default. If that needs to change, it is unlikely that users will be aware of that fact in time to change their schedule, so either SCHED needs to be rerun by operations with the new value, or the observe deck needs to be edited. Users in the vast majority should not worry about this parameter.