I think it's entirely fine for them to not include Vecna Unleashed in the giveaway. It's the most recent and probably drives a lot of revenue still, and though the pricing of expansion packs in the store is a major head scratcher sometimes I get why you wouldn't want to include it in the giveaway even though it's relatively cheap to buy with points compared to Isle of Dread or even the Forgotten Realms expansions.
I think the problem is offering a discovery elixir as a clearly inferior substitute. Either don't give anything to people who already have it (yeah, people will complain, but you're giving something away for free so it's not like they can legitimately say you're being greedy, and the people who care won't be placated by a discovery elixir) or give something of actually similarly commensurate value. Or at least, significant value.
Discovery elixirs are pretty low value, all things considered (depending on how many named loot chests you hit in one elixir, that's probably still not even a single item on average at most unless you're zerging short quests that are likely easier to farm). If you hit five named loot chests and reroll each one (which may mean skipping named loot which does appear) you have basically the chance to get *one* named item.
For the single metric of named loot, discovery elixirs are basically an additional quest completion for every three quests on elite/reaper (33% loot rate vs. 10% loot rate), meaning that you would need to finish three full quests to get the equivalent of just running one quest (rounding up) of "bonuses" from the discovery elixir. Now, other in game boosts are often also a single metric and have limited real world return, obviously, but if you compare that to, say, a Sovereign II Experience Elixir (6 hours of 50% experience), you're getting a "bonus" quest completion every two quests for six hours (ignoring, also, that it applies to some but not all other sources of experience- I've definitely popped a sovereign elixir before using an Otto's box before, for literally a *million* experience points bonus which is a very good return on investment for a consumable experience boost).
Yeah, named loot does have some limitations that make a discovery elixir more valuable, like if you reroll chests or are trying to outrun ransack for a specific piece of named loot, but I'd still place the value at no higher than 400 points (comparing against rerolls) even in an optimized farming setup, which does start to look pretty bad. There are some really specific situations where it matters (farming food for sentient weapons, for example) to boost your named item drop rate, but it clearly doesn't even come close to the value of an expansion pack, at which point you just invite negative comparisons.
