How do I get a lost solar system back from a vassal?

A = Empire with border friction with me
B = Empire that's new to the neighborhood, just hit space age, and are my protectorate.

B has kind of a mouth on them apparently, and started shit with A. I didn't realize that protectorates weren't like alliances, and I didn't even realize B was at war with anyone till I saw ships that didn't belong to them in their system.

I immediately wardeced A that was at war with B, but before I could send some ships over, A took B's one single planet, dissolving the empire completely. One interesting side effect of that now is this. Notice anything strange about my embassy list? Yeah.

Okay, whatever, deal with that later. The other issue is that after I vassalised the fuckhead (A) that picked on my single-planet newbie (B), I now have a vassal (A) with a single planet inside my border. That planet wasn't in the negotiation options when the war started with (A), because he hadn't taken it from (B) just yet, he took it about 10 seconds after I officially wardeced him. But now an hour later, the war is over, and there's this eyesore of geography stellagraphy going on. In the Communication menu, there's no option to ask a vassal for a planet.

How do I fix both of these? I'm missing an embassy slot because the game still has it allocated to an empire that no longer exists, and how do I get this planet back they stole from my protectorate? I'm not planning on recreating the protectorate, I just don't want my vassal owning a planet right in the middle of my borders. What do I do next?