The headliner glue doesn't dry hard. It stays gooie.
You need to plant the vinyl to the fiberglass.
Contact glues work better for that purpose.
Try using a hairdryer on hot to soften the vinyl and see if you can stretch it that way.
Home Depot has got metal clips, like close pins, with rubber covered tips. They work well for clamping the material.
You may need to get a rubber roller for pressing the vinyl firmly into the glue.
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