Hulk Hogan and his family were "really upset" about the death of ex-WWE star Vader -- with Brooke Hogan and Nick telling us the wrestling star was a big part of their childhood.
"He was always a really nice guy," Nick told us at LAX ... "It sucks. My dad had actually wrestled him a couple of times and they had become friends over the years."
"We got up today and we heard the news and it was kind of somber.