Rend Sever and MediaHound, had you tried ZeroHour's fix? It's post
697 here.
And when you run it, be sure to go to "Scheduled Tasks" in the AdminCP and run the vbGoogleMap cron job to refresh markers.xml
You can see if it has bad HTML by getting <forum root>/markers.xml on your web browser, and you can find out where the page execution bombs out with Firefox Tools > Error Console.