If i am reading your post correctly, you installed Win7 on your C: and now want to copy that to your USB drive so you install Vista?
If this is the case, you can do it, but you won't be able to dual boot unless you manually modify the boot.ini file (not recommened for amatures. Whatever you copy the to USB drive will just be there as data, nothing else.
When dual booting, always remember to install the oldest OS first and then the newer OS.
What you will need to do is install WinVista on your C: and then install Win7 on another drive or partition. Unless you are going to keep the USB drive on all the time, I would not recommend using it.
Generally external USB drives are for backup and storage only, not installation of an OS so I highly recommend not using it for your Win7 install.