Executive summary: After a quick
reduction of the VIMOS imaging data from the first commissioning run, the
photometric zeropoints are found to be approximately:
The gains of the two CCDs used on this run were found to be:

The data from the first commissioning data was overscan corrected, bias subtracted and flat fielded in the usual way. Only the imaging data (2Kx2K subraster) was reduced. The image section [1:50,*] was used as the overscan region. There were only a small number of bias frames available for the imaging data. Almost all the flats taken were used, despite the large number of star trails and outoffocus stars in several of them. They were combined using a median which should get rid these defects. A better set of flats will be generated in the future incorporating some of the deeper imaging data (with the sources removed) as well. 
To determine the gain, statistics were
computed on a number 100x100 pixel subrasters of all the images. For each
subraster, the average (in ADU's) was compared to the standard deviation.
The relation between signal (S) and noise (N) in electrons in a blank (sky
dominated) section of sky should be:
Where R is the read noise (in electrons) and g is the gain (in units of e/ADU). Gain is the conversion factor between e and ADU the equation in ADU looks like:
Note that the gain does not factor out of this relation. This means that if one plots N(ADU) vs S(ADU), it is possible to determine the gain and the read noise. At low signal, read noise dominates, and can be simply read off the graph. At high signal, the read noise can be neglected the gain be determined by fitting. The slope of the relation is still one 0.5 on a loglog plot, but with offset due the gain. The above equation becomes:
or taking the log: 
The figure at right shows the noise in ADU plotted against signal in ADU for a large number of subrasters. The blue curves show the fit. The points lying above the curve are derived from subrasters which are not all sky (ie, the have some objects in them) and therefore have a higher standard deviation. 
The zeropoints were determined
using measurements of flux from the Landolt standard stars. I have assumed
that all such observations were taken under photometric conditions. Because
of the small range in airmass of the observations, I have used the measurements
of the airmass coefficients given at http://www.eso.org/paranal/sv/html/data/photom.html
They are summarized here:

The figure at right shows the zero points determined for each star plotted as a function of Landolt magnitude. The means for each filter are also shown. Here, the two channels have been averaged together, which may not always be appropriate. The equation describing the zeropoint explicitly includes the gain. 
Caveats

Send comments/suggestions/problems
to
Stephen.Gwyn@astrspmrs.fr