Hot tub cover repair tape
In my experience, a typical spa cover has an expected life of usually 4-6 years, some only a couple years, some in excess of 10 years but usually 4-6 years. As the foam core slowly gets heavier and or the vinyl ages and gets brittle, the extra foam weight on the hinge, stresses the hinge and causes it to start to tear, especially if you have a lifter that hangs the cover by the hinge, causing additional stress to the hinge.
To answer your question, there is little you can do to avoid the inevitable. You may be able to milk it along a bit longer with multiple layers of quality duct tape or similar, but I wouldn't spend any significant money trying to repair it, especially if the vinyl is aging and getting brittle. Sewing old vinyl is like sewing cardboard, it just gets weaker, if you try. Vinyl patches aren't any better. Also with older vinyl, removing the foam core, may allow the vinyl to shrink and you may not get the foam inserts back in the vinyl bag and or have problems with the zipper. I would suggest you consign yourself to the inevitable fact that you are likely to be replacing the spa cover in the near future, unless you don't mind having a two piece cover that butt together at the former hinge. Sorry, that is all I have for you.