What makes Seras Victoria special?
Here is what makes her special.
Unlike other vampire who run around just killing random people and do what the want, Seras works for Hellsing, they have a reputation for dealing with stuff like the supernatural. Not to mention (as far as we know) She was the first person to be turned by Alucard(if any other people have been turned they were never mention nor brought up in Hellsing's brief run).
He considers her a miracle cause she is most likely the only person who Alucard has never killed(via bites anyway). Or transformed, everyone else is good as dead.

She is special cause she is Alucard's pet, his side kick and almost as dangerous as he is.