Too often I have had images that VPHOT couldn't process because it couldn't get a correct plate solution from PinPoint. Sometimes, it appears, PinPoint fails totally, other times it gives clearly erroneous results (after all, most of us have charts). Last night was another example. I had V, Ic and B images of SN 2012aw and PinPoint failed on one of the images (Ic) and gave incorrect values for the other two. I tried the images at home via MaximDL and got the same result. I worked hard for those images and was darned if I wouldn't be able to report their values (the SN was clearly visible next to the galaxy, so I knew it was there).
I deleted the images from VPHOT and uploaded them to Astrometry.net which solved them in a bout 2 seconds and produced a new FITS image (with header amended to comment out the PinPoint WCS values and put in the correct ones). These I downloaded from Astrometry.net and uploaded to VPHOT which processed them with no problem.