Get Iphone Data Network Statistics
This is not a answer to your question, but some interesting information on what can eat your data plan. (even on standby mode) !

So if you used Google maps (and did not turn it off) on your Vacation, there is a possibility to consume 100 Hours x 22 Mb = 2200 Mb



As for monitoring probably this would suit you.

My Data Manager – Track your mobile data usage and save money


  Description
  
  My Data Manager is a popular app that will help you take control of
  your mobile data usage and save you money on your monthly phone bill.
  Use My Data Manager every day to track your data usage and ensure that
  you don’t go over your monthly data bill and incur unnecessary
  charges.