I also wanted to add my two coins on the Vex:
-As it turns out, there’s actually a couple of instances in Destiny 1 in which you run into enemies on Mercury: during the second part of the Sunbreaker subclass quest and during Sparrow Racing (Shining Sands track). In both cases, the collective encountered is the Hezen Corrective, which would indicate that Mercury is under the authority of the so called Hezen Lords, and that the Hezen Protective is likely also present at other key locations.
-For comparison, the other SRL tracks where the Vex appear, on Venus and Mars respectively, have units from the Aphix Invasive. As we know, some “Eliminate The Target” public events at these locations consist on shooting down a Minotaur from the Aphix Invasive, which does confirmed their presence in both planets long before the SRL. The track on Venus is located on Maat Mons, away from the Ishtar regions, while the track on Mars is at Meridian Bay.
-Then there’s the issue of the effectiveness of the different collectives: the Virgo Prohibition seems unable to claim back territory form the regular Cabal forces, while the Sky Burners that attempted make a beached on Mercury in the Vertigo crucible map were swiftly disappeared in such way that their equipment was left intact. Based on what we know, the Hezen Corrective seems to be the most likely culprit given their confirmed presence in Mercury and this would denote a vast difference in effectiveness between it and the Virgo Prohibition.
-I’m inclined to think that the problem is that the Virgo Prohibition might be too-defense oriented: in Mars they seem more concerned in protecting the area around the Black Garden and the areas both above and underground Freehold. As quick reminder, let’s remember that the conflux in the Scablands is connected to the tunnels that you enter from the Buried City. Also, the infamous Praethorians that guard the entrance to the Vault of Glass also belong to the Virgo Prohibition, with the rest of the Vex in the area being part of the Hezen Corrective.
-On the other hand, the Hezen collectives seem to focus on areas already under Vex control or areas where large scale construction is undergoing/needed. Individually speaking, the Hezen Corrective seems to take an offensive role, while the Hezen Protective takes a more defensive stance. In the case of Venus it is mentioned that the Hezen Protective’s Minotaurs are given construction roles as assigned by Sekrion, The Nexus Mind in charge of integrating Venus into the Vex network. The Vex Sacrifices public event that only appears in Venus could be a different kind of construction role which the Hezen Corrective carries out, for instance, in the Ishtar Cliffs one of the spots for the aforementioned Public Event is a Vex Warp Gate that seems inactive on the Fallen side of the area. We could assume that the goal of the Vex sacrifices in that location is to restore said gate and possibly use it to reclaim that side of the area.
-The Endless Step is one odd area though: the Grimoire card from the Hezen Protective claim that it’s under the control of that collective, but when you actually go (Gate Lord’s Eye mission or patrol) it’s defended by the Hezen Corrective. However, during the first part fo the Nightstalker Quest, you are engaged by the Hezen Protective instead. In that said mission, the large Gate at the top of the can now connect to the Black Garden, which rather suggest that after the destruction fo the Black Heart, which resulted in the Black Garden to return to Mars in the current timeline, it became possible for other Vex Gates (possibly restricted to other large/main gates) to connect directly to it.
-There’s also the Sol Divisive which seem restricted to the inside of the Black Garden. Oddly enough, the Undying Mind which looks like a Sol Divisive Hydra is classified as belonging to the Sol Primeval (the Precursors), who are said to have been its ancient creators. Perhaps the same is true for the Sol Divisive as a whole?
-On the other hand, the Sol Progeny is not limited tot he 3 vessels sued by the Black Heart, but also includes the Overmind Minotaur at the Prison of Elders, and IIRC also Gate Lord Qodron himself. On a related note, it is not specified to which collective does Gate Lord Zydron belong to, but I’m inclined to think that he could also belong to the Sol Progeny and essentially be a collective that includes only high ranking Axis Minds. On a separate note, Qodron is depicted as having the same black smoke coming out its frame as the Sol Progeny vessels that the Black Heart possessed, which makes me inclined to think that his erratical behavior might be the result of being the first test subject to be possessed by the Black Heart.
-Leaving the Precursors (sol Primeval) and Descendants (Sol Imminent) aside, that leaves the Aphix Invasive, which is all around the place and are particularly known as Qodron’s followers who are fought in the Reef. Much like the Sold Divisive which has a distinctive physical trait (vines), the Aphix Invasive frame can be distinguished by the spikes on their frames unique to that collective. If I had to take a guess, I would say that they are also an offensive-based collective, seen on areas away from the Black Garden in Meridian Bay and away from the Ishtar region in Venus, not to mention their out of the way presence in the Reef.
So, what about the collective in Destiny 2? Much like the unsustained claims that the new Fallen House was a “House of Dusk”, I saw that some sites called them the “Outer System Vex”, a term that would fit their presence on Nessus and Io, but became outdated when the Curse of Osiris put this same collective on Mercury. Also, much like the Aphix Invasive, they have physical differences with the rest of the frames form other collectives.
All that being said, I think that ultimately the problem is that like much else form Destiny 2, the enemies have been oversimplified, which is why we no longer have such subdivisions and basically each race is now a single group without distinctions (with the exception of the Cabal and later the past/future Vex), a fact I find quite disappointing as I found very interesting such distinctions.