Yes, what you are describing is a simple dual boot solution. Boot from your Vista DVD, and choose the D partition to install. I would have left D unpartitioned, and the started the VIsta DVD, but either way should work.
install XP first then Vista. Works fine. You will by default get a boot up screen that offers "Windows Vista" or "Previous Version of Windows". Realize that each will Boot up as its partition as drive C switching the assignment. I added a third partition E that holds the common programs, media, archives, etc and made symlinks or modified my default locations of each OS to reference E.
the only problem, more an inconvenience, is that some programs require a double installation due to licensing or registry usage. That and mail is a bugga boo. Pick one or the other side and stick with it. Ive tried like hell and cant get windows mail to export its files to outlook or visa versa (contacts, etc, yes, data, no).