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.
HN user
uckelman
Our experience has been that the two things you're asking for---a shallow learning curve for learning to use a game's UI and a UI customized for each game---aren't compatible in practice. We arguably have the latter, a UI customized for each game, right now and it generates no end of confusion because users expect the UI not to change completely between game X and game Y. If you give the module designer broad freedom to alter the UI, then what you get are differences in game UI between games which are down to the designers being different people more than differences between the games themselves.
I'm one of he VASSAL developers. We're flattered to see VASSAL featured here. Should anyone have any questions, I'd be happy to answer them---either here, by email (uckelman@nomic.net), or in our forum.
Since the site is directed mainly at non-technical users, we don't dwell on that much. To answer your questions: It's written in Java, it's a desktop client.