Fortunately, after nearly two hours of troubleshooting, I was able to get things to work in Windows 7 RC x64. But, I would like to still report this as a bug.
When playing the WoW Minis mod in vassal under normal settings, whenever I tried to open a new map (which should add a new panel below the normal window), I would see it pop up for a split second and then completely disappear.
As it turns out, setting the vassal program to run in the Windows Vista Service Pack 2 compatibility mode gets it to work. Windows 7 recommended Windows XP service pack 2 compatibility mode, but that did not work.
This shouldn’t be a problem for now, but it may become one in a few months when Windows 7 releases.