Related to this question, I'm wondering also whether using Vassal on a netbook (=small screen) is a good idea. Are there any tricks helping in such a case, like zooming in/out etc.?
No, using Vassal on a netbook is not a good idea. The standard resolution on most netbooks is 1024x600. While this isn't a bad thing for reading most websites, consuming video, etc. it doesn't work particularly well with the high resolution game boards in Vassal. The problems you will face will differ from module to module, but some of them will be pretty standard.
Most boards will require scrolling. - You can help mitigate this issue somewhat by enabling scrolling with the keyboard arrows. The keyboard is a much better input device for quickly moving the board slightly up, down, left, or right than moving your mouse or built in touchpad to the scrollbar to click/drag the gameboard in the proper direction. You can enable this feature within File->Preferences for each individual game.
Additional Windows always overlay (are always On Top of) the main window. - This will only be an issue if your particular game requires multiple open windows, and each of those windows needs to be open constantly. You can attempt to give your self a little more room to see each of the individual windows by using the zoom controls for each particular window. Fit to Width/Fit to Height will fit the graphics within the current dimension of the window. Additionally, you can shrink the Windows Taskbar to a 5 pixel bar (it won't show the icons of your open programs) which should give you another 20 pixels of screen real estate.
The Vassal Toolbar isn't dockable or configurable within the module. - This means that you cannot remove elements from the toolbar and make them their own floating windows, hide them, or dock them to other windows or other locations on the application. This means that when the module designer creates a toolbar icon, it will always be displayed above your game board. (Example: I play D-Day Dice. This module has 7 buttons that open other windows, 2 that roll dice, and the standard Vassal toolbar controls. Because I cannot configure the toolbar, it takes up two rows of space (about 90 pixels). You can modify the game module yourself to remove unwanted items from the toolbar with Edit Module. Refer to the Help Guide within the Edit Module dialog for more information.