I posted recently about replacing the vinyl window completely. Replacing the stitching alone would be a much easier job.
My post is
here
I don't really go into detail about the actual sewing process, though. The sewing awl that Healthservices posted is exactly the same one I used. The only thing I did differently was to use the heavest standard sewing machine needles I could find rather than the huge needles the awl comes with.
Sailrite's youtube video on repairing shoes is perfect for giving you the technique it takes to sew with the awl for this project, too. I think I found it on their website, but you could probably search youtube directly as well.