There is a file under "Documents" if you're using Windows 10, and in there is a folder for Venetica. Within that folder is a file called "settings-user", which is an .ini file.
Open this file with Notepad and locate where it says fullscreen = true, and change true to false to run it in windowed mode, as there's no in-game option to change it.
There is also a file that is in the game installation folder that is called "settings", which is also an .ini file. Open that and locate "vsync_enabled = true". Change that to false, which does have a bit of an effect.
Also, if you get screen tearing, go to your graphics card Control Panel and turn vsync on, which seems to work better than the game option listed here. Doing the second trick did help, though it added about... 10 frames, I guess. Still, it is a bit better doing this.