I have Doctor at 80% but I still don't get the option of getting a medical tour from Dr. Troy in Vault City. He doesn't ask me to get an example of Jet either. I think I struck a conversation with him before my doctor skill was 75%, which I believe is the minimum skill level for the tour. Have I burnt my chance forever now? The only thing Dr. Troy offers me is healing, nothing more.
You didn't by chance blackmail him, did you?
I tested several scenarios and blackmail was the only way that I was able to have the dialogue not appear. It also prevents the dialogue to complete the Jet quest from appearing. You could take the tour first, then blackmail him.
Whether or not you are a bit of a jerk to him in the dialogue doesn't seem to matter. Merely suggesting blackmail also does not seem to have an effect.
This was on a character with 2 Charisma, -10% speech, and 4 intelligence. An intelligence lower than 4 means that he won't talk to you at all. (that should rule out those stats having a positive effect)
(scenarios 7 and 8 are the only two which cause the medical tour option to go away)