If they are shrunk and pulling away, even if you glue, they will pull away again unless you remove the whole length and reposition. The piece will be shorter and will have a space somewhere. I chose to have the space under the combing bolster where the gap doesn't show.
If they are lust loose, they can be adhered with 3M double sided molding tape. It comes in different sizes. For your molding you need the 7/8 inch width. That comes in 15 foot lengths. You have to remove the old adhesive from the fiberglass and the back of the molding. A heat gun will help with the vinyl part and maybe will help with the fiberglass. It can be a lot of work. Folks have stated that the 3M adhesive remover is helpful but I haven't tried it.
New molding is expensive at about 10 dollars a foot from Grady. Search for the older posts, there is a lot of info and possible sources for the trim.