Since this page was empty I thought I would jump in and add a few things I learned while making the De Bellis Vassalus miniatures mod.
Make your counters as small as possible while still being legible. It also greatly helps to make them a nice even size, like 30x60 pixels or 40x40. This makes it much easier to figure out the ground scale in pixels to the inch. When I started DBV, my counters were 73 pixels wide, equating to 40mm in game scale, this made figuring out how many pixels equaled an inch a pain. I now make my counters in nice round numbered sizes. 50 pixels to the inch is much easier to work with than 34.5 lol. The line of sight thread can be used to measure distance once you get your pixels to inch number, but since it cannot be seen by other players I found it necessary to create a ruler piece.
Rotating pieces seems to be the biggest memory hog, use as small a rotation number as you can get away with. I started out with free rotation, dropped that to 96 facings, and have now dropped it to 48 with little effect on gameplay but less memory use from rotation. Pivot is a handy feature for miniatures mods. It took some trial and error to get it to match my rotation values but I got it eventually. My ruler rotates 48 facings, my pieces have a fixed pivot ability of 10 degrees and -5 degrees. Then it took some messing around to get the pivot point to match the exact corner but those numbers will vary depending on counter size.
If at all possible, use the default blank VASSAL maps instead of images. Miniatures games seem to require much larger maps than most of the boardgame modules and if you use a png map its gonna be a pretty big file. I also try to keep my terrain pieces as small as possible in terms of file size.
Hope some of this is useful, opening up the DBV mod in edit mode may also prove useful.