I have installed that way - ( from memory,when you get to the screen that allows Upgrade or Custom , choose Custom ) you get a screen to select the partition to install on - just a question of making sure you click on the right one.
Suggest you make an image of your existing Vista install first and save it somewhere - another partition or external hd. Also run off the recovery cd from the imaging program - needed to boot in and restore the image.
I am not expecting you to need it right now, but it's smart to have image backups always.
You can use the free version of this program - it's excellent Macrium Reflect FREE Edition - Information and download
Hope it helps