I would recommend you start with the game for which you know the rules well (EastFront in your case?). Vassal has a bit of a learning curve so you don’t want to have to be thinking about rules too much while you find your way around the software.
I suggest Napoleon. It has no cards and has a pre-defined setup. I created most of the vassal mods, and I put a solitaire option in all of them, so you could play it alone (offline) to get comfortable with the basics.