Other mistake: When Vlad is in the armory of the monastery, he is standing directly under sunlight. This should have caused his skin to burn immediately and give away that he's a vampire. (00:52:05)
Other mistake: Arabic inscriptions can be seen on the golden desk, but the letters aren't connected like normal abjad. This is likely a computer rendering error since some editing software don't natively support foreign characters. (00:17:10)
Chosen answer: Actually, what the master vampire told Vlad is that he will return to mortal after three days if there is no feeding. If Vlad does feed, he will become a new vessel for the dark power, a new offering to the dark. Nothing was said that Vlad would be trapped in the cave but rather a scourge on the Earth. It is true that if Vlad feeds, the master vampire would be set free. After that, the master vampire can take revenge on the one who turned him. A new game of revenge would then be played between him and Vlad.
That's kind of weird because I remember the Master Vampire saying the same thing about Vlad staying in the cave forever too.