That 500 MB is used if you connect via 3G or 4G. Not if you connect via WiFi and your BT connection.
There are android apps to monitor the data use via Vodafone, but I can't tell you details about them. I'm sure somebody else can.
I inherited a sony xperia D2303 smartphone & when I rang vodaphone for internet prices they said as a valued old Vodafone user they give me 500mb for Free.
I went on line via my BT Infinity home Hub, found my inbox & sent an email successfully, then I downloaded & installed the free 465MB "co-piot" sat nav or mapping.
What I want to know is:-
1. What is that 500MB for?
2. does that 465MB comes out of the free 500mb the gave me?
3 Also does my going online, emailing or browsing affect that free allowance?
4. How can I check frequently if I exceeded that allowance.