So I just finished Ring-a-Ding-Ding by using the speech option to get Benny up to the presedential suites alone, where he explains his plans. Afterwards he left, and the quest complete thing came up, and shortly after I was attacked by his guards.
After this I went back to Swank at the counter who told me that Benny fled to his suite, but checking his suite I can't find any sign of what happened to him, and Yes-Man has disappeared too. What do I do now?
I'm afraid that Benny lied to you. You might want to see if the intercom is working in the presidential suite, you should hear from him.
But regardless, Benny has scampered off with the Chip. To find out more you need to fully explore his suite. There's a back section to it with a very interesting character who can (dare I say, must) reveal everything to you.
This is where I am in the story, actually (though I didn't convince him to go up, my speech skill is atrocious and I didn't fancy a shootout in a casino armed with only a pea-shooter). Now I need to go after Benny, and it sounds like a doozy.

Itkovian