In this specific case the shomer in question is a young man named Yakov (Dave Davis) who is still recovering from a traumatic event involving his younger brother.
Needing the money to pay for his medication, he agrees to spend the night watching over a deceased man, but what he is horrified to discover is that there is entity existing in the house that not only threatens the spirit of the dead man, but also his life and future.
For his directorial debut, Keith Thomas most definitely found an aspect of Jewish faith with a high creepiness factor, as The Vigil tells the story of a shomer: a person who is hired to serve as a guardian for a person between the time that they die and are buried as a means of comforting the spirit that remains.