Is the gaseous entity for "obsession" a less evoled crystal entity

[Gary7]

But our heroes would have had every motivation to prepare the bait so that it would be easily penetrated. In contrast, a sealed starship proved impenetrable, so that the creature had to channel itself through an actual opening - which led it, to its detriment, to a cabin occupied by a Vulcan. It probably mistook the entire innards of the ship for inedibles because of that... Remarkably, it did not even penetrate the apparently less than airtight doors of the cabin (see the crack below?) to seek for tastier stuff. So even "faking" a barrier should be of help.

I do appreciate the fact that yes, under typical circumstances you'd expect that you'd need the bait to be easily retrievable to lure a prey. But then there's also the element of allowing enough of the bait to be sensed yet still not reachable. We see a sealed glass like container. We don't see them "pop the top" and fully expose the hemoplasm. Visually, it appears that the creature can penetrate glass (which is in essence a form of liquid, albeit an extremely slow moving one--perhaps the gaseous nature of the cloud allows it to penetrate this). While spacesuits have transparent panels in the helmet to enable vision, I expect they're made of a special composite that is completely solid, though.

It can, but it didn't. For eleven years it hid. For untold years before that, it hid. It would be absolutely crucial to know why this was, and where it hid, and why, when and how it decided that it was time to feed on humanoid blood.

Like HIV, it could be dormant or isolated to a small region, then suddenly someone gets infected who travels about the world (patient zero, as in Gaëtan Dugas), spreading it around. Here this creature may have been dormant but is then jostled awake by the proximity of humans. After a good healthy feeding, it's preparing itself to spawn, to then spread itself all over the celestial populations. Remember, it was dormant on a planet with no sapient life forms present. It may have wiped out whoever lived there, then went dormant and waited for the next arrivals.

And um... if we DO kill one then we know how to kill another, btw!

With the Borg, that's the exact opposite of the truth.

An exception. The Borg adapt rapidly due to their technologies. A creature not equipped with technology can't really adapt well against inherent vulnerabilities. A human being can grab a shield to protect its fragile body, but once the shield is knocked away, the original vulnerability is in play.

Intriguingly, the Vampire Cloud appeared to be a master of adaptation, too - fortunately, we saw no signs of a collective mind that would let other members of the species learn from the death of one. But if we kill our potential Judas goat, then the herd cannot be killed. At the conclusion of "Obsession", our heroes had no idea how or where or when to find further Vampire Clouds. That's the crucial step in killing them: once located, they can apparently be lured to become opaque to antimatter blasts for the duration of feeding on a blood bait. And a blast somewhat smaller than the impracticable one used in the episode might well be devised, once our heroes learned more about the creature. (Except now they can't.

The only adaptive mastery I recall is its ability to hide from sensors completely, or to appear at will as dikironium to sensors. Why that element? It may be something that evolved over a long period of time. It knows beings that seek out this element are warm blooded and potential food. It can also remain inert or travel at warp speed. When it attacked the ship, it looked for an opening and found it, then started attacking people. After a quick snack, it then sought refuge to hide, which was the ventilation system (it probably has the ability to detect air currents). Once in there, all vents were closed so it was trapped, but then that meant a limited air supply for the crew. They then flushed it with radiation to drive it back towards the number two impulse vent, which worked--the creature left. It wasn't so much adaptability as simply survival tactics.

While very clever and apparently intelligent to some degree, the creature may have absolutely no means of communicating in ways understandable to humans, it refuses to communicate because the target is beings who are its food, or it simply can't communicate at all--it is a visceral creature that is just very clever about hunting for food.