Depends on which incarnation of Star Wars we are talking about.
If we are talking about the original Star Wars movie, and discount the lore that was retconned in the sequels, then killing Vader shouldn't have actually hurt the Empire that much, as in the original film Vader wasn't anything particularly special. Given the way the imperial officers treated him as just the muscle and talked down to him, it's safe to say that he was replaceable, and was nowhere near as important as later films made him out to be to the Empire's security. So it'd be a small victory, mostly meaningful to Obi-Wan and no one else. Little implication for the war or the galaxy at large.
If we take the original trilogy as a whole, then killing Vader is basically a death knell for the Empire, as he is basically the second in command who intimidates people into doing what the Emperor wants, and basically has to be present for any plan against the Rebels to succeed. With Vader dead, the Emperor will be hard pressed to find an adequate replacement, and the Emperor certainly won't be able to train any new apprentices in time before said apprentice is strong enough to be worth anything.
For that matter, there's no reason for Obi Wan not to fly the death star trench run himself.
If we go by prequel trilogy lore where Obi-Wan is shown to be a pilot, then yes. He would probably be one of the best pilots present.
If we go by the original film, or the original trilogy, there is nothing to imply that Obi-Wan was a pilot. It was implied that he was merely a warrior on the ground in the clone wars.