I think he was deliberately taking the hit to provoke a reaction from Vader. He'd seen how much it meant to Vader to reach out to Luke as a father, to want them to be together as father and son. So he believed that Vader had it in him to choose Luke over the Emperor, and he let himself be tortured while Vader watched in order to force that choice.
I think there are a couple of questions about RoTJ from the story point that I still don't fully understand (so many years later). One is (a new one in this thread?) the whole conversation that Yoda says to Luke that Luke doesn't require any more training.
For reference - here's the script -
YODA
No more training do you require. Already know you that which you need.
(Yoda sighs, and lies back on his bed.)
LUKE
Then I am a Jedi?
YODA
(shakes his head)
Ohhh. Not yet. One thing remains: Vader. You must confront Vader. Then, only then, a Jedi will you be. And confront him you will.
So far, I'm of the view that Yoda knew that he was dying and couldn't further train Luke, so he was being encouraging. But added to that, I feel that Yoda probably knew (due to his Force perception) that the Force is taking steps to "balance itself" and it's time for Luke to take over the good fight and for Yoda himself to exit, stage left. i.e. Yoda has faith that whatever that is happening is the right thing.
Also - not quite sure what Yoda means by "confront Vader". To Luke it meant fighting and killing Vader and he says that he won't be able to do that and to that Yoda responds that then, the Emperor has already won. But on seeing the rest of the movie (and how it all turns out) perhaps Yoda was saying confront him with the Truth. (Which will set the Universe free?). I am not entirely sure that Yoda truly knows that this is going to happen - I still think he does it at least partially on faith.
Then the other thing is what is Vader's plan and why he takes certain actions - when he tempts Luke, it is clearly suggested that he thinks Luke and him could kill the emperor. But when the time comes and Luke charges the emperor, Vader intercepts the saber strike. If he had not, then the Emperor would have died (the lightsaber is inches from the Emperor when it's stopped by Vader's block) for sure. Why not let the Emperor die at the last moment? Or even why not simultaneously strike the Emperor? I don't have a clear answer to that.
Similarly - what exactly is Palpatine's plan on "turning Luke" over to the Dark Side. Enrage him by baiting him and then - what? get him to kill Vader?! (I *think* so, but I am unsure) I guess the plan was that when Luke killed Vader the rage, self-loathing, guilt would help turn Luke over to the dark side. (Not to mention sticking to the Always-2-there-are maxim).
And then is Luke really thinking anything when he throws the lightsaber away? Does he really think/hope that he should submit himself and *hope* that Vader comes thru at the last moment? I don't think Luke is really thinking at that point. He's just reacting. So I certainly don't think that it was his grand plan (or spur of the moment plan) that he should let Vader watch his son get tortured, to goad him into action. I think he throws away the lightsaber in order to regain the good-light-Force equilibrium which he's in danger of losing as he gives into hate (a bit) during his fight with Vader. From then on, things just happen too fast for Luke to actually think it thru. He appeals to Vader for help, not out of some plan but as a natural appeal to somebody for help.
As for the bolded part of your quote,
Christopher - why do you think Luke has seen how Vader wants to reach out to him? Not the lets-band-together-n-kill-the-emperor speech surely. I don't think Vader wants to bond with his son but it's more of a powergrab - more like use Luke to kill the emperor and become the Sith-lord with his son becoming the apprentice.