Don't ask introverts "are you busy tomorrow?” Probably, they are. Arranging matching socks. Staring at ceilings. Flipping old books. You know, important stuff. It entirely depends on the rest of the information you’re about to give them. Make it count, they're not wasting time.
“Maybe I’ll come” - I’m not coming
“I’ll let you know” - I’m not coming
“I’m not sure yet” - I’m very sure that I’m not coming
“I’ll try and come” - I won’t try anything
“I’ll think about it” - I won’t think about anything, I’m not coming.