I cannot comment on this as i have not experimented with Vista's drive management enough to make a broad statement...
All i know is i wasn't impressed with it and went back to using my old standby partitioning tools to get the job done.
That being said it may work perfectly fine for you.
If you do not need anything off of D: then delete the partition.
Then re-boot and see if Vista will merge the free space for you.
I'm old school. I like to do all my partitioning/formatting outside of the OS itself which is why i suggested the alternatives.
There's also a FREE tool called Paragon Partition Manager Express
Here that may suit your needs...