function TForm1.GetSelectedMarker: TMarker;
var
   pNode: PVirtualNode;
   pData: PVSTMarkData;
begin
   pNode := VSTMark.GetFirstSelected;
   if assigned(pNode) then
   begin
      pData  := VSTMark.GetNodeData(pNode);
      Result := pData^.Marker;
   end else
      Result := nil;
end;

var
   vI:      integer;
   vNodes:  TNodeArray;
begin
   vNodes  := TVirtualStringTree(Source).GetSortedSelection(True);
   for vI := 0 to length(vNodes) - 1 do
   ...
end;