This has been happening to my boat for a while. I stripped the vinyl off and took it to a local upholstery shop to have them resewn. The issue I had was that there was barely enough material to sew back, so the guy did the best he could. Eventually the split again, usually due to people walking on the cushions and putting a lot of pressure on the seam. I gave up for now, and have been stripping the vinyl off and replacing it with a solid color piece as the get too bad. I got to where about half the boat, is just the one color stuff. Eventually, I will have to have the whole boat redone, maybe next year. I think the guy three of my cushions for like $150 or $200. But he was a car upoholstery guy and he didn't use stainless staples, and they rusted and failed eventually.