Eddie Griffin -- Vegas Wedding Came Out of NOWHERE

Eddie Griffin and his new wife had ZERO plans to get married on the fly in Vegas last week -- his wife tells TMZ, they just woke up and decided out of the blue ... it was time to tie the knot.

Griffin's bride Nia Rivers tells us, it came out of nowhere on Thursday -- all of a sudden, they couldn't go any longer without being married ... so the couple beelined to a Vegas wedding chapel that same day.

Eddie and Nia -- who just bought their 8-bedroom dream home outside Sin City -- got engaged in June ... inside the revolving restaurant on top of the Westin building in Atlanta.

They have an 18-month-old son -- and Nia tells us, as soon as they find the right sitter ... the newlyweds are heading straight to Fiji for their honeymoon.

Or Bora Bora. They can't decide.