Here's how. Not sure if it fully gives back your EFI partition, but it's good enough for Boot Camp and I think firmware updates will be able to use it.
1. In Vista, open up Computer Management and format the 200MB empty drive that was once your EFI partition as FAT32 (You can also do this with the Vista Disc, in case your Vista install is corrupted).
2. Reboot off of the Mac OS X Disc
3. Switch to Console
4. Type in "diskutil list" and note which partition is your Macintosh HD. In my case it was /dev/disk0.
5. Type in "fdisk -e <partition>". However it is important to note that you will need to insert an 'r' before the disk device name. For example, in my case I typed "fdisk -e /dev/rdisk0".
6. Type "print". This should list the partitions of your Macintosh HD. You should see a FAT32 200MB partition, followed by an HFS+ partition and the partition you used for Vista. If this does not match up, stop here and retry fdisk with another disk or give up. Note the start and size of the partition and write them down.
7. CAUTION (fdisk is very dangerous): type "edit <#>" where number is the number of the 200MB partition used for the EFI. In my case I typed "edit 1".
8. CAUTION: You will be prompted for a System identifier (id) for that partition. Type in EE, which is not in the list provided by the help. Once again, I am retyping the last line to ensure there are no typos. Type in EE, which is not in the list provided by the help. Choose the default [n] for the next question about editing. Enter in the identical start and stop numbers you recorded in the previous step. In my case the start was 2048, although I don't recall the end.
9. CAUTION: You are done, but the changes have not been written. If you might have made a mistake, cancel now and exit and nothing will be lost. I would type "print" and ensure that the starts and stops match the previous print done before the edit. If you as sure you entered everything correctly, type "write" and fdisk will write the new partition table. Please do not proceed with this step if you are not sure about the procedure.
10. Reboot into hard drive Mac OS X.
11. Use Boot Camp to remove Windows partition.
I have done this TWICE and it works, but be very careful. Good luck.