Rs.50 pack is normally a top-up pack. It gives top-up of around ₹39 talktime with 28 days validity. But it doesn't extend your service validity.
However, in your screenshot look at the "only for today" tag.. Vi always gives curated offers for individual customers. So my guess is that they are giving full talktime of ₹50 along with 28 days service validity for your number. And this offer is only for today.
That's why it says both "service validity" as well as "no service validity"