Is there a way to do it? I have already bought windows vista so I would like to install them on my Mac mini. BootCamp 4.0 says that it requires windows 7 but is there any way to install vista?
Just because it's not officially supported doesn't mean that it won't install or work. Give it a try, this might well work... But seriously though, Vista is the worst OS to come out of Redmond ever! I would advise you to think about purchasing Windows 7 or 8 if you need to use Windows with your Mac.
Vista isn't that bad (with SP2). It's just a resource hog and didn't run well on the hardware that was shipped with most computers in 2007. It should run fine on any recent machine. Why you would purchase it over Windows 7, though, I have no idea.