Does he just say "I will not speak with your kind, vampire" --> "Goodbye"?
There does seem to be a refusal dialogue that should be filtered by if the player is a vampire, but it isn't. However, it also is filtered on whether you're carrying the potion, so maybe this will work:
player->additem p_sinyaramen_UNIQUE, 1
So if you're holding that item, I think he'll talk to you.
Edit: Actually, since you originally posted this in the CS forum, maybe you wanted to know how to fix him. I'd do it by editing his "Greeting 2" dialogue ID 63326584685031353 and 23288195233207216141 so they are filtered by PCVampire >= 1.
Edit 2: Just checked, and it looks like the MPP actually does fix this dialogue by adding that check, so it would be interesting to know if you use it. If it's not working, maybe post your load order, there might be a conflict.