Ah, but Vader was drastically weakened since ROTS, and not nearly as flexible. Plus Vader isn't a saber expert, really. Unless I've missed something, he was never shown as being a master of any form, even though he dedicated himself to form VI. Obi Wan could dictate the rules of battle, and vader could only hope for the best.
You might be right. I think it is form V. A typo, my bad. But yeah, form V isn't exactly a very sound policy, unless you're charging hordes of stormtroopers. It's emphasis is on reflecting and redirecting of blaster shots. It's rather like a perverse version of Form III, lacking the actual tight-knit defensive moves.
But yeah, good point... Obi Wan outgunned a much more powerful version of Anakin. Old man Vader doesn't stand much more of a chance.
This is easy. We already saw Old Master Ben Kenobi (ANH) fight OT Darth Vader. He lost on purpose; Vader had no advantage. Now let's put him up against Young Master Obi-Wan Kenobi and let's see what happens. The desire to win, youthful vigor, and just as much ability (if not more, for Old Ben Kenobi might've put the Force on a shelf to gather dust for a while) and we've got an easy win for General Kenobi.