How can I repair my sagging shower door?
I think my approach would look something like this:

Dismount the door assembly from the shower wall frame and move it to a safe (soft) workspace.
Pull the glass entirely free of the door frame.
Pull the vinyl sleeve out of the door frame.
Clean all parts thoroughly with bathroom cleaner, then alcohol, to remove all residue.
Very gently attempt to flex the door frame back to straight. Use spacer blocks under it and go easy. If you kink it you're done.
Reinsert the sleeve into the door frame, then the glass into the sleeve. If necessary, use a dilute soap solution as a temporary lubricant.

Once it's all back together you'll need to assess whether there's enough friction to hold the glass. If not, take JACK's advice and look at an adhesive. I'd probably start with something soft but gooey, like rain gutter caulk.
Of course, any decent glass shop can probably reassemble this for you (possibly with new seals) for a reasonable price.