I occasionally have issues with sims not being able to train skills. Well, training isn't so much the problem: they can train them just fine, they just don't learn anything by doing it. Even after sending them to take a class, they're still at 0 progress. However, these sims are not former vampires...they're brand new sims out of CAS, or townies in a fresh town. But, if it looks like a duck, walks like a duck and quacks like a duck, chances are good that it's a duck. So I wonder if these situations are somehow related.
Personally, I've never unvamped a sim. So I'll ask any of you who have: does this happen every time? Obviously this doesn't happen every time with new sims or townies in a new game. I can exit without saving and start over, using the same sim, and it won't happen the next time or the time after that (occasionally it has happened a couple of times in a row). So saving before drinking that little potion might be a good habit to start getting into.
However, I've never been able to find a way to fix my problem once it manifests.