Is it possible to install Windows Vista 64-bit on mid 2010 27" iMac w/ Mavericks?
You can do it without using Boot Camp, just make this few steps:


Install rEFIt.
Resize your disk so you get some free space for vista.
Create a new partition from that free space.
Boot up your Mac with the Vista install disk inserted.
Install Vista onto the new created partition.


If you need a more detailed How-To/Tutorial/Manual/Assist (or what ever you will call it), have a look here http://lifehacker.com/5531037/how-to-triple-boot-your-mac-with-windows-and-linux-no-boot-camp-required