We have no idea. I would assume the Traveler would be able to do so, given the power of paracausality over the Vex, we just never had a chance to see it in action. All of the machine worlds in existence right now are when the Traveler isn’t really capable of doing much at all, and hasn’t been since they were converted, and had other pressing matters than to zip around the system reverting the damage done.

But I’m assuming if it were back in action and had the freedom to do whatever, it could probably change it all back. It may possibly depend on the damage done though. Like, I have no clue the extent of the Traveler’s terraforming powers that would be needed to unfuck what happened to Mercury - completely converted on the surface, and completely hollowed out, and also nearly destroyed by the Almighty. Could the Traveler, even at its prime power, completely revert all of that? No idea. But I (Mercury Nine) sure did show us a vision of us exhumed while holding a “pale trowel,” a line that’s keeping me up every night, so you understand the kind of stress I’m under. What do you mean bestie? You want someone to come gardening again?

But Theia is still only known about because this entity was part of the Solar system, so it’s very unclear if this sort of a thing exists anywhere else outside of Sol, even if they appear to have originally come from somewhere outside of Sol and got captured in the gravities of the forming planets (and the Sun). According to II, they weren’t able to find any other besides what was going on in this system, including the shambles that happened at the start. Although we definitely need more than one lore tab about Theia to make any judgements about that. It’s interesting also that another one failed to form in the asteroid belt.

I don’t think it’s ever been confirmed that the Vex have a chunk of the universe converted. Obviously we don’t know much about the wider universe in general, but whenever the Vex appear to exist elsewhere, they’re not described as ruling the universe in any way and it’s been that way since at least the time of the Witness’ species (who were aware of the Vex and used their technology, as best we can understand). Vex technology seems to be known to other species and others have encountered them or were aware of them (like the Hive and the Psions for example). So they’re around, but they don’t seem to really affect the world as much as you’d think, besides the occasional machine world here and there, which is honestly quite intriguing.

Speaking of machine worlds, the Nine do not die if something like this happens. We know this because of Mercury. The Nine related to it (I) is very much alive, around and speaking to us normally. Mercury is by far the most thoroughly Vex-converted planet we know about. The Nine associated with is also very aware of this but does not appear to be directly negatively affected. Probably the same reason for it not being affected by the planet being gone; its gravity is still around. That’s at least from what we know right now. There could be something going on here that hasn’t been revealed yet.

Honestly, I do think they could be. I genuinely don’t see anything that would prevent that from happening. After all, there’s precedent that AI, even if it is super advanced AI, can achieve personhood enough to be rezed (Felwinter), so there shouldn’t be many barriers to a Vex individual.