This was first detailed by Indian Express. Under the Rs 399 pack, Vodafone India is putting forth 90GB of 4G data to its clients for a half year.
image from twitter

There are no certain announcement by Vodafone if the offer is for the new or existing Vodafone client.

Fundamentally, a blessing from Vodafone to its clients. Note: This offer could be for selected circles , so you may need to check before reviving.