You can use Microsoft Virtual PC 2007 SP1, it's free, and it will do what you want.http://www.microsoft.com/windows/products/winfamily/virtualpc/default.mspx
I used it. It's pretty self-explanatory, just set all the default settings to Vista when it asks.
Although the performance isn't great, you'll get the general idea. Although it would be best to just make a partition, and install it. It will run better, and install faster.