Why is your
iPhone 3G locked to Vodafone? Are you not out of the contact lock in period? Will they noy unlock it for you?
Frankly, i am not a fan of these iphone data plans that are offered by Airtel & Vodafone. The plans are a total ripoff. When you are on the so-called iPhone data plan you are paying more money for data. If you had a Vodafone Mobile connect data plan with the same data limit then you pay less than the iPhone data plan. How does the same amount of data become more expensive bases on the device I use when the volume of data is the same?
In answer to your question, here is what I'd do..
First option: Jailbreak your iPhone. Get a regular Vodafone Mobile Connect or 3G Plan and use it with your iPhone. I have a jailbroken iPhone 3G which I use with my Vodafone Mobile Connect data plan. I pay Rs 399 pm for unlimited 2G data. There is no Vodafone 3G in AP so I use a BSNL 3G SIM for my 3G needs.
Second option: This is more technical. If you'd rather not jailbreak then you could access & modify the Carrier Configuration file on your iPhone to change the APN. This would allow you to activate and use the Vodafone Mobile Connect or 3G plan on your iPhone. In the Vodafone India Carrier Configuration file on the iPhone the APN is configured as "iphone". This is the reason you cannot use any other data plan with the iPhone from Vodafone. Modifying the APN should allow you to use a different data plan on your iPhone.
Thanks for your detailed answer. I have asked Vodafone to unlock my set but they said they cannot do that.
I would like to go with your second option. Let me know how to proceed and whether there is any risk?