[Grumpy old man]

usually if they are dried out and brittle it's time to replace them , But if you want to try to save them you can try this old school fix . Vinyl is a petroleum product .
1 ) Clean the affected area with NON ethanol reg gas on a rag .

2 ) once dry use a hair dryer on low heat to soften the area using light pressure try to smooth/ stretch the area flat then apply Vaseline Original Unscented Petroleum Jelly using a dry rag . let sit for awhile then reapply heat and vaseline again if it stretches out and flat enough use contact cement to reglue it . it's a process and if your project is just too dried out your looking at replacement .

Tip : when heating the affected area keep your fingers in the air flow , If the heat is too hot for your fingers it's too hot for the vinyl .

Good luck !