As long as you have Vista disk attached at instalation of W7 you won't need the disk.
Now you have 2 options here depending on whether you wish to keep the Japanese version.
To keep it install W7 by booting the W7 disk and choose custom install and direct it to the second drive.
To get rid of it, boot the W7 disk but this time use the disk tools in W7 to do quick format and install it on that drive.
Keep in mind that if you chhose to install to the second drive while Vista is connected, you will not be able to just remove the Vista drive as the bootloader will be installed on it.
You would then need additional help to recover W7 after the removal of Vista.
These instructions are based on the assumption you are getting the upgrade version.