Vodafone - Excessive Billing of Postpaid

I had a travel to Germany from **/*/** to */*/**
On **/*/** I activated the International Roaming on my postpaid Vodafone no ********** for a period of * days from */* to */*. Enclosed snapshot of plan and confirmation message recd from Vodafone. I paid **** R for the same (which I assumed would be charged in my bill , being a postpaid customer).
I left India on **/* - Working fine
Reached Paris **/* - Internet working fine. From **/* night when I reached Germany, intermittent signal and from midnight no signal. My workplace was ** kms from the hotel where I was staying and I was talking to my family once every night using whatsapp call feature while connecting to hotel wifi.
When I came back to Mumbai and checked at the store, they coolly tell me I have to pay Rs *****/-. If this is true, then the whole concept of international roaming becomes a farce. This is simply unacceptable. I have been writing since last * weeks to Vodafone - but response is we have sent a request for waiver and it is rejected. Need help to resolve, and may I make it sure that I will not accept that this amount is payable as signal did not pickup at all and you cannot use so much data in such a short time