Ending is unnecessarily confusing
This is one of those films like The Exoricist that is great and terrible for the same reason: the film never outlines any rules or logic. Now a film can work with no exposition, and this one does work about 95% of the runtime. But without any idea of how vampirism works the plot unravels at the end, looking very arbitrary and thrown together.
Vampires can't die, they just grow too weak to move and become immobile husks. Got it. So then why do they all come alive at the end? Why did Miriam freak out looking at Sarah's jugular wound? Did she not realize that vampires are immortal? Why did she suddenly age so rapidly? If the fall has no effect why did it appear to "kill" (rapidly age) her? And if if vampires can die like humans (as the monkies died in the lab), why didn't Sarah die from massive blood loss and why didn't Miriam die like the male monkey?
Does the book explain any of this?