The strange number is JMPs internal representation of a datetime value (actually the nr of seconds since a reference date I keep forgetting...).
VLine() seems not to have an optional label argument. But you can populate the graph with vertical lines using Add ref line() in a for loop. Use Summarize() to get lists and matrices of the labels and dates from the data table. See example script below.
// Make example table
dt=New Table( "Test",
Add Rows( 6 ),
New Column( "Group",Nominal, Character, Set Values(
{"Group A", "Group A", "Group A", "Group B", "Group B", "Group B"})),
New Column( "Data", Numeric,
Set Values( [1, 2, 3, 2, 3, 4] )),
New Column( "Date", Numeric,
Format( "d-m-y", 12 ),
Set Values([3166214400, 3166300800, 3166387200, 3166214400, 3166300800, 3166387200])),
New Column( "Label",Nominal,Character,
Set Values( {"A", "B", "C", "A", "B", "C"} )));
// Make list of dates and labels
Summarize(dlist=by(:Date),dlist=mean(:Date));
Summarize(llist=by(:Label));
// Make graph
gb=Graph Builder(
Variables( X( :Date ), Y( :Data ), Overlay( :Group ) ),
Elements( Points( X, Y, Legend( 1 ) ) ));
//Add vertical reflines based on dates in Column date
for(i=1,i<=nrows(dlist),i++,
report(gb)[Axis Box(1)] << add ref line (dlist[i],Solid, "Purple", llist[i] ));
I hope this helps.