Agree with the above, but also check the vinyl around where it pulled off. If that vinyl has delaminated, take a razor blade and trim to where the vinyl is still firmly attached. The G-Flex will adhere to both, and the vinyl is there to protect the inner layer from UV and abrasion, which the D-ring patch will do just as well.
Another alternative is Stabond, which is the adhesive Mike Yee recommends for gluing his vinyl patches into either an R-lite hull, or a boat being reoutfitted in which the vinyl has been torn off in the process of removing older patches: http://www.mikeyeeoutfitting.com/
The bond of the vinyl to the ABS may be weaker than either the bond of the vinyl to itself, or the bond of the ABS to itself.
I've found that the vinyl layer can be easily skimmed off the ABS with a single pass of a sharp chisel held at a low angle. Easier than sanding by far.
John Sweet, boat builder emeritus, recommends removing the vinyl from the ABS before installing skid plates. Maybe we should be removing the vinyl before critical D-ring patch applications too. With the vinyl gone, adherance of G-flex to the ABS should be terrific.