I recently got vampires and I was experimenting with this (No mods) and it's a game of chance, kinda like how just plain old "WooHoo" can end up with a baby 1-2% of the time. So if you have a little friendship with a sim, 98-99% of the time they will not let you drink from them asking permission. (With Get Famous, if you have more fame and the sim is a fan, this likelihood turns into about 60-70% from what I've experienced). If you have a lot of friendship with a sim, there's a 5% ish chance they will not let you drink from them when asking for permission.
A theory I have, but haven't tested enough is that like some sims like fruitcake, while other sims despise it, some sims are totally fine with vampires drinking their plasma and other sims are not. I believe it's something the creators put in the game to make it more interesting. I don't think it's a bug and I personally like it that way.