Well, it could be a different saber other than Vader's, but I think JJ is a little smarter than the naysayers give him credit for. I think... I probably shouldn't say what I think. Maybe Abrams and Kennedy are really more blaster kind of folks than they are saber kind of folks.
OR a rumored dream sequence/flashback with Vader somehow.
OR a rumored dream sequence/flashback with Vader somehow.