The two things that will be important are 1)default boot drive? and 2)which is the default OS you generally which is answered by Vista installed after 7. Since 7 was on first then Vista the 7 mbr information may have been effected or you now have the Vista drive seen as the default boot device in the bios.
If you were booted in 7 Vista would be E or another drive letter while 7 remained C. When booted in Vista you first look in the Disk Management tool to see what letter is seen for 7 there. You can also change that to another letter if needed and associate 7 to that in the change settings/entry based settings section in 1.7.2.
When first installing Vista and XP was XP the second OS to be installed or last? When installing XP after Vista and now 7 you need to repair the newer version's mbr entries in order to make it bootable again.
With Vista on last however you should be able to add 7 right in without problems when selecting the Vista/LongHorn option followed by naming the new entry to what you have there plus the drive letter seen in the DM. You may have to delete the first entry and create a new one along with verfiying the drive letter for seeing 7 associated with the actual drive letter you set in the DM.
The image here shows how 7 was associated to C while booted in 7.
Once the new entry is created while booted in Vista, Vista being C there, you verify the letter for 7 and follow through with the association of the drive letter there followed by looking in the view settings to see if that is being shown exactly the same. When everything matches then try booting into 7.
Further problems getting 7 to load would suggest a need to repair or rebuild the 7 mbr information by first unplugging the other hard drives and booting to the repair tools on the 7 dvd you made. When going to install XP and Vista with those drives added on later the 7 boot information was likely trashed in favor of Vista then becoming the default host drive or the boot order for drives saw a change.
Once you have 7 booting on it's own without a need to reinstall but repair the mbr you then replug the other drives back in to see the new entry in the Vista BCD made knowing that 7 now boots on it's own. A return trip into the bios to set the default boot device to Vista would be the thought unless you want the 7 drive set that way and add Vista and XP into the 7 boot loader which is done the same there as in Vista.