A friend of mine is designing a game, and I'd like to playtest it with him. We have no possibility of meeting face to face; also, finding ±90 minutes to play through Vassal when I'm at home is quite difficult. However, I travel quite often by train, and then I have much time. The problem is, I have then only a very slow Internet connection (using my mobile phone). Do I assume correctly that this should be enough to play through Vassal? (I assume that it sends only info about my move - no graphics etc.)
Vassal is pretty low bandwidth - as you correctly guess, the graphics are all part of the pre-downloaded game files, so in play it only sends information about which pieces you moved and where to. It's on the same order as using a text chat app, and pretty favourable compared to a web page.
Obviously you'll need to download and update the actual game files in advance, as those can be larger.
(There's still enough bandwidth usage that you could notice the cost, however, if you're on a pay-per-byte plan for your mobile.)