I haven't done anything with Lemmini for ages, however if I recall correctly, it's written in Java correct?
Java is platform-independent, so I'd expect that Lemmini should not care whether it is running on Vista vs XP vs Linux vs Mac vs what-not. So maybe it has nothing to do with Vista, in which case you might want to check whether all the required files are there. (Again, not having used it for ages, I don't remember what files you need.)
Also, does Lemmini at least give you some sort of error message? The error message may give clues as to what's wrong.