Of course he had a plan.

Just going up to Vox and the other Vee's and saying he'd willingly be Vox's prisoner would be too obvious. Vox would question him. The other two would be suspicious. He didn't need that. He needed things to go smoothly.

So after leaving the Hotel to speak with Rosie, he came up with a plan. Not a word to anyone, he went to provoke Vox. Of course ending up in a fight with him. He was a bit surprised when Nifty and Husk showed up but they were pretty much his minions, and Nifty was, well, Nifty.

After purposefully losing, Alastor struck a deal with Vox. He would be Vox's prisoner and Vox would not to lay a hand on Charlie. And Vox made the deal, happily.

Course this did ensue public humiliation for Alastor but he dealt with it. Knowing the outcome of his deal would be worth it.