I have the My Verizon app for the Xoom, but it only tells you what day you're in in the billing cycle and how much data you have used from your total allotted amount. It doesn't give you a breakdown of how much data each app is using.
I went to the My Verizon site and it does give you a breakdown of how much data is being used each day, but again, it doesn't tell you what app is consuming data.
I'm personally looking for a 3G data monitoring app that will show what apps are using data. I've read elsewhere that you can sideload 3G Watchdog using the apk file. Other suggestions is to turn off automatica synching with your Google account.
The VZ Mobile for tablets widget would be great if it worked properly. Switching from 3G to WiFi renders the widget inoperative and the user cannot re-logon (from the app). The only solution is to turn the Xoom off then on while on a 3G connection. Another problem arises when trying to logon from the app. In my case, the app grabs the information tied to the number of my phone, not the number assigned to the Xoom. So, I am forced to either logon to the website to retrieve the info, or routinely power the Xoom on/off while connected to 3G for updated info. Pretty useless.