I have a sim who just started dating Caleb Vatore. They're friendship is maxed and the romance bar is about 25%. Three days into dating, Caleb calls my sims saying that he has something to ask her and...
I went ahead and said yes and luckily he did turn her. I had a vampire/mortal couple in another save with max bars and such a thing never happened. I do love random surprises like this though. =)