I have no idea how much data Vlingo uses. It's available in the App Store, you could try checking the page for Vlingo and see if the info you want is there.
Oh man... That's a bummer. I've been wondering what was taking people so long. Oh well. I don't know why someone hasn't just made a good replacement for the 3G now.
I figure you could make something that injects into MobileSubstrate. Now that the SDK gives access to things like the addressbook and iPod it shouldn't be out of reach. Maybe just more work than is worth if the 3G will be such a small portion of iPhone owners by next year.