The games are automatically saved every 10 minutes, as well as when you exit Virtual Villagers. While it's true that there is no in-game facility for backing up your game(s), there is a fairly simple way to do it manually:

Pause all of your games so that the games will suffer no ill effects from the passage of time. Using Windows Explorer, go to the directory where Virtual Villagers is installed and look for files named Isola**.ldw (where ** is a number). The single-digit numbers are your "primary" savegame files and are numbered 1-5 (corresponding to the player slots). The two-digit numbers are the "backup" savegame files, and are numbered 21-25. There is also a file Isola0.ldw which contains settings.

Copy those files to a safe location until they are needed.