"Trip in vain" really means an enroute interruption that makes it useless to continue the trip.
IE: you're flying from your home airport to your destination via a connection at a hub. You board flight #1 with indications that all is good. However, by the time you arrive at the hub, flight #2 is delayed to the point that there's no point in you making the trip. You can claim "trip in vain" to be returned to your home airport and your ticket refunded. If this was a return potion of a round trip ticket, then it's not a "trip in vain" as the trip to your destination was completed, you're just flying home.

Your situation:
- if noted while at the airport, would be a normal IRROPS rebooking/refund
- if noted after you got home will likely be considered a no-show. If it was part of the meltdown, then I'd hope DL would rule in your favor and not call it a no-show.