Nothing pertinent is kept in the registry. If you wish to get the new value of the progress you can check obj.value. If you want the value before the adjustment, you can check the value with the onadjustbegin event (instead of the onadjustend event).
<script>
window.onload = function (e)
{
var vprogress = new RGraph.VProgress('myc', 83, 100, 0);
vprogress.Set('chart.color', 'red');
vprogress.Set('chart.adjustable', true);
vprogress.Set('chart.margin', 5);
vprogress.Set('chart.tickmarks.inner', true);
vprogress.Set('chart.label.inner', true);
vprogress.Set('chart.units.post', '%');
vprogress.Set('chart.width', 100);
vprogress.Draw();
RGraph.AddCustomEventListener(vprogress, 'onadjustbegin', function (obj) {document.getElementById("output").value += 'Value before adjustment: ' + obj.value + '\n';});
RGraph.AddCustomEventListener(vprogress, 'onadjustend', function (obj) {document.getElementById("output").value += 'Value after adjustment: ' + obj.value + '\n';});
}
</script>