VASL is that adaptation of the VASSAL engine to the ASL game system.
Those who know Advanced Squad Leader (or ASL) know it as the finest tactical wargame of all time. One of the best selling military boardgames ever, ASL depicts WWII combat at the level of individual squads, guns, vehicles, and machine-guns with a degree of detail never achieved in another game. Those who enter the world of ASL often find that its complexity means that opponents are difficult to find, particularly for people who don't live in major population centers. This is where VASL comes in.
VASL is a computerized interface for playing ASL over the Internet. It does not replace the need to own the boardgame, but it does allow any two players to connect and play each other live over the Internet. Unlike games played over Internet chat sites (IRQ's), you are able to see and manipulate both the maps and counters on the VASL screen and transmit this information instantaneously to your opponent. Play by email or connect to the online server, where you can view other games being played, share information, chat, and compete in online tournaments. It's a club for those without a club, and it's changing the hobby.
(Quoted from the VASL Website)