I searched for help on the ToyotaNation forum, since the Toyota Matrix and the Pontiac Vibe are near-identical twins, and found a report that that plastic part became unavailable years ago. (So I suggest that the part that Dorman &co. list as only for models up to 2007 should in fact be considered to have superseded the part that they list for 2008 models.)
Further prodding showed that the rear end of the heat shield would in fact stay in place (but not very securely, of course) if I pushed hard against the bolt, so I thought I would try epoxying the threaded part back in place, but I made the mistake of trying to unscrew the bolt from it -- and sheared the bolt.
Time for a redneck repair: I threaded an 8" zip tie through an appropriately sized washer, up through the hole in the heat shield, through the plastic part (parallel to the strap), back down through the hole and the washer, and through a suitably sized nut (3/8", I think, and then pulled it tight. Everything seems firm.
https://drive.google.com/open?id=1JdacG ... 8NGZdBJRdg