Not sure why it did not work in your case but i am in USA here and have a Vodafone number. I had never activated International roaming but after i landed in the US, Vodafone started to deduct 99 Rs per month from my prepaid account. I had to keep my balance proper to ensure that i am able to pay that amount every month.
Well, if you go out of India then no sooner you run USSD code (or pick a call or send an SMS), Vodafone activates international roaming and deducts 99Rs!
Checking balance itself is USSD code! So now you know why Vodafone kept deducting 99Rs!