Hot tub cover repair vapor barrier
Get protection. If you're lucky enough to live in a sunny location, a quality UVA/UVB vinyl protectant is mandatory. Apply as directed 3-4 times a year. DO NOT use 'Armor-All' brand protectant as it contains petroleum distillates and damages vinyl long-term. A quality saddle soap or marine application protectant is good, or try '303' spray.
If you use Bromine, keep the levels to a MINIMUM, as the gas trapped under the cover will "orange peel" the underside and make it brittle.
When you shock weekly, it's very important to leave your cover open/off for a jet run cycle (15-20 min.) so expelled gases can escape. This protects the underside of your cover, as well as clears your water for clarity.
Inspect cover for cuts/pokes through vinyl, breaching the inner plastic vapor barrier. This plastic bag envelopes the foam core, keeping it dry. As soon as a small hole is made, the foam cores are so dry, they will wick in moisture slowly and get heavier and heavier, causing extended wear and tear to your cover. Repair the plastic liner with good quality clear postal mailing tape.
Repair cuts/holes in the vinyl with a good quality vinyl repair kit. Most cuts can be glued from the back side by adding a small scrap over cut and putting tape on front side, pulling it back into shape. A good vinyl glue (VPL) can be found at most auto parts stores.
Lastly, you can remove your foam cores (they all zipper out at hinge) and flip them over and put them back in to reverse the 'sag'. This should be done once a year, or twice a year if your tub is 7' or bigger.