It's not a Visio issue and there is currently no way to manipulate or export the full topology graph in MARS. I've heard rumors it is on the "Road Map" but nothing concrete.
The full topology graph is generated by MARS and is useful for instantly giving you an idea where incidents or policy violations are coming from or destined to. It's not designed to replace the topology diagrams generated by CiscoWorks or HP Openview.
Hope this helps.