Question: Wordperfect 5.1 (for DOS) doesn't work without modification under Vista. How can I get it to work (as well as possible) under Windows Vista?

Answer: 
I would suggest creating a Virual Machine running DOS. You can download Virtual PC for free [here](http://www.microsoft.com/windows/downloads/virtualpc/default.mspx). You will need a copy of DOS to do the install.
Maybe DosBox does work. It works fine with most Dos Games and Programmes. It's free and still beeing improved <http://www.dosbox.com/>