Octagon Mirror

My friends have grown up in Vietnam. They had these little wood-framed mirrors in the shape of an octagon (I think) hanging above the doors in their house. When I asked, they said it was a mirror to reflect the evil spirits of the house. What are these called? Is it part of a particular religion? Or simply a question of culture? Is this a correct statement of what they are for? Where can I buy one?

This is called a "Ba-gua mirror. Common in Taoism and Feng Sui Very. Chinese hanging above the door (outside the house) to push the bad things like negative energy, bad luck, or "spirit" that you just mentioned.

Ba'gua the mirror You can buy them in any city in China with stores that sell Chinese stuff
they keep the bad chi or Joss. Hang it in the front window of a room or to put it on the edge of the window up.
There are many more, I know the culture Chinas.
The octagons are a set of three lines in various combinations of Broken and solid and one on each side of the octagon, they also used the I Ching (Book of Changes) the 8 directions (cardinal and secondary), they are Reference points for Tai Chi, they are used in Taoism and Wu Dong and some other cultural, spiritual and mystical traditions.

Ba'gua the mirror You can buy them in any city in China with stores that sell Chinese stuff
they keep the bad chi or Joss. Hang it in the front window of a room or to put it on the edge of the window up.