What do you mean by "wildcard"?
By wildcard, I mean that it's the type of exploration encounter that is extremely open ended. It could be central to the campaign (your Troll Boots) or completely insignificant (the characters ignore it or can't figure out what anything does). Not even the DM knows. The Deck of Many Things is a more straightforward example of what I consider a wildcard.
Which is also perfectly fine and interesting. It also means the vats can be used against the PCs at their own peril. Also fine.
You seem to have completely missed my point here. Vats containing liquid don't really stand out or inspire wonder. While the contents are indeed wonderous, the players will only ever be inspired by it if they experiment and figure out what they do.
OK. But that tree doesn't really lead to decision making, which is the primary activity the players do.
Of course it does. They can certainly investigate the tree in a similar manner to the vats.
To illustrate, imagine if they found a grove of crystal trees, the sap of which corresponds to each of your vats.
However, even if they don't investigate, they're still inspired with a sense of curiosity and wonder. The crystal tree is distinctive (assuming crystal trees aren't all over your world).
If the players assume that the vats are just water storage for the dungeon denizens, they may walk right by them and forget about them. They probably won't forget something as distinctive as a crystal tree, because no one's going to mistake that for a water tower.
I wouldn't call the vats a challenge. They aren't something to overcome. They, like everything else, simply exist in the game world. It's up to the players whether to interpret them as a challenge or not. Just like it's up to the players to decide whether to engage with them.
From the player's perspective, sure.
From a DMs perspective, some things are placed there to challenge the players. If you place a puzzle lock on the door, that's a challenge. The players are of course free to ignore the door or to find an alternate means past it (passwall) but it nonetheless exists as to challenge them. That it exists within the game world like everything else doesn't change that.
Walking away is fine. It's an interesting choice.
I don't get what you mean by a waste of time, though. Maybe you can explain.
I strongly disagree that walking away is an interesting choice. If you think the players ignoring the exploration encounter means you've created a successful encounter, I've got news for you.
If the players can't figure out the liquids and simply grow frustrated and leave, it's been a waste of time for them. It's like adding pointless filler content or worse.
Let's assume a fairly inexperienced group, just for sake of illustration. One reckless guy quaffs the concrete potion and dies a minute later. The fighter sticks his sword in the reincarnation potion and nothing happens. They unseal the black ooze and another party member dies in the ensuing combat. The players decide the DM placed these vats here just to mess with them and walk away.
Of course, that's not the only way this encounter might work out, as evidenced by how it played out in your own campaign. If at least some of your players were experienced with D&D, then it's likely that they were familiar with this trope and knew that if they were careful and stuck with it, it could pay off well.
Under different circumstances or with different players it may not have worked out as well. Sometimes the Deck of Many Things spices up a campaign and takes it in interesting new directions. Other times it wrecks it. I've seen both outcomes.