This should be easy to fix -- go either into the Vista that works or the W7, and use the DualBootPRO that you have installed there.
Look at the drive letter provided by Windows Explorer for the drive containing the new Vista set-up -- use the Manage OS entries tab and check the Add New OS entry box (I'm using VistaBootPRO and the precise names of these elements may have changed in DBP) -- provide the name that you want, use the drive letter provided by Explorer in that Windows setup, and select the appropriate Windows version (Vista).
Oh and btw, there is one possible (but rare and unusual) potential issue, due to the nature of the Vista/W7 pre-boot environment -- the pre-boot environment does not always use the same order of drives/partitions that appears in Windows Explorer, so that the drive letter element can sometimes be a little temperamental, and you may need to try creating multiple boot entries using several different drive letters. You could try creating several entries for C:, D:, E:, F:, I:, X:, and see which one works, if the standard procedures fail to help you that is (which is unlikely). You can create several entries named drive C, drive D:, etc -- after you know which one works, you can easily use DBP to remove those that you don't need.
Having said that, I'm not sure what your intentions are for the old Vista setup -- if you are planning on keeping it as a backup, then the above should work ; but if you're planning on deleting it and recovering the hard drive for other purposes, then the better solution may be to simply use your Drive X in the Disc 0 position -- so that your boot environment were completely independent of the old Vista setup (which will not be the case if you retain the old Vista drive in the Disc 0 position).
If you continue to have problems, can you please then explain what your plans are for the old Vista setup, and what your plans are concerning that hard drive ? This will be helpful to let people find the best solution for your boot environment