Why didn't Vlad the Impaler make his wife drink his vampire blood just when she was about to die in Dracula Untold? It would have set him free from the curse, and afterwards he could have drunk her blood to turn himself back into a vampire.
One possible answer is that he did not want his wife to be suffer from the curse, and she also tells him that they will meet again in another life (at the end of the movie he meets her "reincarnation").
Another possibility is that with your solution he would not be able to remain vampire, his powers were almost gone, there could be a chance that he would become mortal again if he did not feed in that moment (the sun was almost up and he was quite isolated down there).
He could only turn people when he had full vampiric blood. What he was for those three days wasn't a pure vampire, so he didn't have vampire blood, only the traits. We see a few minutes later that he's able to turn the people who are almost dead back at the Monastery, because he's finally consumed human blood. Also, he likely wouldn't have been able to drink from her then turn her, because she was already seconds from death.