Ok first of all could you please let us know your system specs, that way we'll have a better idea as to what problems you may or may not encounter when installing Windows 7. Alot of people, myself included, have installed both the 32 bit and 64 bit versions of Win 7 with no problems at all. Though there have been some issues for some people. but that is to be expected as Windows 7 is in BETA stages and one should almost expect at LEAST one issue when using Beta software.
Also, where did you obtain your copy of Win 7? And which program did you use to burn the ISO to a DVD? Is your copy of Vista 64 Bit? These are all things that will help us give you the answers your looking for..
You said you've partitioned your hard drive into 2 sections. C: and D: . With Vista being on the C: , alright so your looking to dual boot than? If so and your running a fairly modern system, than you shouldn't have any problems or at least no major issues. All you would have to do is boot from your Windows 7 DVD and do a clean install onto your D: partition. Once the install is done it's thing (hopefully problem free, and in about 20 minutes ideally) you'll have a dual boot setup meaning you will be able to use both Vista and Win 7...
You can do an upgrade if you'd prefer but from the sounds of it a clean install would best suit your situation. And as always, please remember to backup all your personal files before installing a new OS, especially a BETA OS... and not to sound repetitive but as always, if your not comfortable using BETA software than please don't..