Get the Imperial Assault Campaign module for Vassal from http://www.vassalengine.org/wiki/Module:Star_Wars:_Imperial_Assault
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(each expansion pack has two different skirmish maps).
I'm sure you mean each expansion has one map for two skirmish missions.
The core box comes with Luke and Vader expansions, thus 3 maps and 6 missions.
To date (I don't have the Hoth-stuff yet), there are 17 maps, 34 skirmish missions.