I've haven't seen this yet with an active sim, but I've seen this with some unplayed sims that my vampire was trying to turn. My vampire sim offered to turn them, they agreed and were bitten. After a a day or two, they had the animation for turning, but they didn't actually become vampires. I could tell this because they didn't have the vampire symbol in their profile and the option to turn them had reappeared in the vampire interaction menu.
The strange thing is that it's hit or miss. Some unplayed sims were successfully turned and others weren't. If a sim failed to turn once, I was not able to successfully turn them on re-attempt. It either worked the first time, or it didn't work at all. I have to wonder if there's some sort of characteristic that interferes with becoming a vampire. One of the two vampirism-resistant sims was Anaya Jang the other was an auto-generated townie.