What do board game designers (and publishers) feel about this... I assume that it is relatively easy for IP to be ripped off, does the Vassal community self-police this? Or is it more a case of there are no sales lost due to the existence of copycat modules in Vassal since if you actually want to play the board game with friends in close proximity, you'll actually just buy the damn board game :-)
But I could potentially see lost sales of tablet implementations of board games; although those almost always codify the specific rules and logic of the game so again quite different to Vassal.
Board game designers will very often give permission for things like this so long as there's not an official money-generating version in existence, or at least ignore their existence.
I think the most visible case of this happening is the deck-building game Dominion, where the policy was that it was fine to have free versions so long as they didn't use the artwork and so long as they're taken down when there's an official app. (Which has now happened on some platforms and I think is expected to happen on others soon.)
I also suspect that Vassal is so niche that most designers/publishers don't really see it as taking away very much from paid apps, even if those do exist. The people playing on Vassal are generally the kind of people who buy a lot of board games.
Many, many games published these days have a VASSAL module available before they go to press because game designers are using VASSAL for playtesting. In the wargaming world, there are publishers which actively promote the existence of VASSAL modules for their games (GMT and MMP are the two big ones which come to mind, though there are others as well), some which are indifferent, and a few which are opposed. The impression I get is that the availability of VASSAL modules increases sales for publishers which aren't in opposition to it.
Looks like they discourage the use of artwork and text, but dont actively self police.
"The vassalengine.org web site owners do not verify whether or not uploaded modules contain copyrighted materials. However if the owner of copyrighted material makes a request, we will remove a module from the web site."
But I could potentially see lost sales of tablet implementations of board games; although those almost always codify the specific rules and logic of the game so again quite different to Vassal.
Like for example, Carcassonne: http://www.vassalengine.org/wiki/Module:Carcassonne