How well known were the Sith among the general population of the Galaxy Far, Far Away during the events of the original trilogy?
I was a kid when the original film came out. Somehow, pre-Internet, my brother and I knew the "baddie" as Darth Vader, Dark Lord Of The Sith. For me, my older brother was the Internet, and often knew stuff from magazines and books he had read, so I may have picked up the Sith title from him.
Then there's this from the screenplay, available in countless places online:
"Revised Fourth Draft January 15, 1976
INTERIOR: REBEL BLOCKADE RUNNER -- MAIN HALLWAY.
The awesome, seven-foot-tall Dark Lord of the Sith makes his way into the blinding light of the main passageway."
As far as we know, the Sith did exist then (even if Lucas hadn't formally thought of them) because he injects them into the universe later on, but as for the subject of whether or not characters in the first film knew, all we have is that a small number knew about the Dark Side of the Force, but beyond that, it's bound to be conjecture.
In-universe, the word "Sith" was well-known even if its exact meaning wasn't. Four thousand years before the events of the original trilogy, there was a galaxy-wide conflict known as "The Great Sith War", which was essentially the armies of the Jedi & Old Republic against the armies of the Sith Empire.
As for the time of the original trilogy, most people thought of "Sith" as simply some kind of evil Jedi. It was not widely known that the Emperor was a Sith, but as you stated, Vader was a different story. So, for the average person in that galaxy far, far away, Darth Vader - who was a common sight on the HoloNet and in Imperial propaganda - was essentially the "face" of the Sith.