Greg, I think your “left” and “right” backlash numbers are quite high. I posted this a while back:
So my average backlash was only 0.020 - 0.030mm, similar to your “back” measurement.
It looks like your X axis has some mechanical issue, like too much play somewhere or a loose belt or something. Another possibility is that your probe does not make good electrical contact on some sides. Make sure the surfaces are really clean and have no rust etc.
It shouldn’t really matter whether the vise is bolted down. If you have good contact then the force should be very tiny. You need to watch the probe as it touches the vise, it should really barely touch it and not try to move any further.