But what if Vader hadn't been in the throne room?

It would have been a much less interesting scene, that's for sure.

In my mind, the Emperor was clearly stronger than Luke, and Luke would have died. This raises the question of why Yoda would send him off unprepared. He did warn Luke not to underestimate the Emperor, but Luke probably thought that he was in for an emotional battle with the Emperor, not a physical one. When he throws away his lightsaber, it may be as much for Anakin as it is for the Emperor. It's as if Luke is saying "See father, I don't fear this man, and neither should you."

Of course, this horribly backfires, but it also pushes Vader to make a choice.

So if Vader wasn't there, and Luke just happened to be before the Emperor, the Emperor probably wouldn't have even tried to turn him. Remember, he said: "Only together can we turn him to the Dark Side of the Force." The Emperor is relying on Vader, so without Vader, Luke would just be imprisoned or more likely killed outright.