Let's see ... bare-handed, the best thing Spock has is the Vulcan neck pinch. Darth, on the other hand, has the Vader throat pinch ... and he doesn't even have to touch him. I suspect in Star Wars terms, Spock has some Force sensitivity and heightened strength, but not nearly enough to take on the Emperor's right hand man. Armed, the odds even out a bit more, but probably not enough. Vader could probably shrug off a phaser set to stun, but would that palm-deflection trick work with a phaser set to vaporize? That's a lot more energy to be re-directed than you'll get from a blaster.
The simple fact is that Spock wouldn't get in a fight with Vader. He'd observe from the sidelines as others fight the Sith, then advise Kirk on his weaknesses.
Kirk would then press the "OFF" switch on Vader's torso and bed Princess Leia.