Kujan actually tripped Verbal up a couple times
I don't think that "Oh, f-ck!" Verbal lets out when Kujan comes through the door barking 'Who's Keyser Soze?" was premeditated; I think Verbal was genuinely cornered for a second, worried his real identity was out, before he realized Dave just wasn't familiar with the figure, and that he should just go with the flow and build the legend of Soze into his story.
Also, as some spoiler video on youtube pointed out, when Dave is later shaking Verbal for the truth, Verbal blurts out "I AM Keyser!" before it gets buried in the crying and Dave's insistence that Keaton was Keyser.