[samdk]

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.