I also was shocked to see Vader’s ship straighten out, because I remember us all assuming that Vader had died (the first three viewings were all with the same crowd). With his ship straightening out, it is obvious that he survived, and I doubt that all of us would miss that.
I know that I watched the film many times without committing that detail to memory as well. Not sure why, as they would have definitely been 1980s tapes that I was watching. Perhaps I was assuming there should have been more closure as a standalone film (even after knowing the sequels). But I assumed it was one of Lucas’s tweaks when I saw the 1997 version in theaters.