8GB is not for everyone. Its either 3GB for 3 months or 8GB for 3 months and it is decided solely by vodafone on what offer you get. Also you can get 9GB x 3 months if you visit their site's "amazing offers for you" page
I was offered 9GB on Vodafone 499 plan, thus I decided to port from Airtel. Sadly, in the intervening days, Airtel too decided to give me 8GB till the end of the year.
Now that I have shifted to Vodafone, they have given me 7GB instead of 9GB. 1GB plus 2GB (4G), plus 3 GB for the next 12 billing cycles. And upon arguing with the Customer Representative, she offered me extra 9 GB (15GB/month) for the next 3 months.
The offer has not come through and I am still only getting 7GB.