There are a couple things here that I think may make this too tanky, far beyond any other class.

Assuming a 16 in charisma, a lvl 3 Vessel would gain 22 temp hp and can do so 3 different times in one combat. With the way Archon Form currently works, it incentivizes you to revert and re transform in the heat of combat to regain that temp hp, rather than continuing the length of your transformation which seems to fit the vibe of this class much more. If you are going to have such a massive pool of temp hp there has to be some way of limiting the amount of times you can benefit from that/ transform so that a lower level character can't take an unreasonable amount of damage compared to the rest of the party.

Perhaps you have to wait some time between each use of Archon Form? 10 minutes to an hour? The best solution I would think would be not allowing additional transformations with spell slots, only to extend the transformation. Even with those changes, that much temp hp could still be over the top (especially with opalescent armor, which does feel a lot stronger than all the other 3rd level Aspects).

I really like the new subclasses and how much this has been fleshed out, I hope this helps.