OKAY!
So.. I make the partition active, then install the OS X.
After installing I repair the boot with a windows repair disc.
I do the commands as razy said, and then boot into either 7 or Vista.
Then I install EasyBCD and add the OS X bootloader, and then just reboot.

But will I still have the dual-boot with Vista and 7, like I have now? (Triple-boot with MAC OS X)
Like.. will there be listed 3 OS', after I add the Mac OS X bootloader?