I have been reading up on a few definitions. Divination is a method of obtaining unknown knowledge. Necromancy is a form of divination through communicating with the dead. Therefore, does this make everyone who uses Oui Ja boards, all spiritualists and anyone on a vigil who says "Is there anybody there?" a Necromancer?

You are absolutely right Ian.
Nekros = the Dead Manteia = the Art of Divination Nekromanteia = The Art of Divination through the Dead . My classical studies are finally paying dividens.
The traditional meaning is very clearly exemplified by the Witch of Endor summoning the soul of the prophet Samuel at King Saul's bidding. Samuel prophesizes that Saul will be defeated and killed by the Philistines and so it happens.
So technically when we are attending a seance and we ask a spirit insight about the future we are technically using Necromancy.
H.P. Lovecraft, being well read in classic literature, had the "necromancy ring" in The Case of Charles Dexter Ward doing exactly this: summoning the dead back to land of living, giving them a body (so they can fell pain and be tortured, of course) to steal their secrets.
More recently, especially in horror literature, it has taken a more sinister meaning but this doesn't concern us.

Who sad the art of Necromancy was dead

I think Tolkien was the first one to use the term "Necromancer" in the more widespread meaning used today but I could be wrong.
As with the term "Satanism" the word changed of meanings at least three times during its history.