I recently ran an online PBEM game (actually, it was play-by-Slack) in which I was both game master and player. We often had battles that needed to be done when one of the combatants was offline. I used the NIST randomness beacon ( beacon.nist.gov/home ) for card draws. Basically, I specified a future time for the draw, and then converted the random NIST output to a series of cards. Because my announcement was timestamped, it was clear that I was unable to cheat. The time only had to be a couple of minutes into the future. Obviously, the idea would work for dice rolls as well.
I then wrote some java code that automated this, and made a little app that converted battle-times to independently verifiable card sequences, with the cards displayed graphically. Any two players using the same battle times are guaranteed to get the same card sequences, using NIST as a trusted third party, and the sequences cannot be predicted in advance. The communication of the battle times needs to be timestamped, to prove they have not been retrospectively chosen, but that is not difficult. I’m not sure if Vassal can do this but a lot of chat software can.
I will probably add something similar for my current Vassal mod, but I thought it might be useful in other mods, too. Ideally, I would like to see it incorporated into Vassal itself, if possible.