Hello Caramia and welcome to the eForum
We're always here to help so please feel free to ask as many questions as possible during your stay.
It sounds like you are using the usage display that comes with the Vodafone Mobile Connect dashboard, VMC, control panel, this is not always the best way to monitor usage. The VMC usage counter gives an overview of usage, but for a more accurate account you would need to register for Vodafone My Account.
My Account can be found on the Vodafone website as linked, once you've registered your dongle number (can be found by inserting into a phone and dialing *#100#) you will be able to view billing information and calculate your monthly data totals.
Please keep us posted on your progress and further questions.
Thanks
Reidar
eForum Team