The ZVD update from Sprint only runs to full completion on stock recovery.
You can flash it in xionia, after turning of signature verification, but it will not flash the modem. After you flash in xionia and boot for the first time, stock recovery will be installed. Then if you flash the update a 2nd time, you'll not only have the ZVD ROM, but the ZVD modem as well.
If your purpose is only to go back to ZVD so you can go to rooted ZVJ, then you don't need to worry about the modem being ZVD. Your ROM will still boot, you just won't have any data, until you get ZVJ installed (ZVJ ROM/ZVJ Modem)
Alternatively, you could just install stock recovery from the beginning, then you could flash the ZVD update.zip once and everything would be installed in one shot.