You can use the POR 15, but I would shoot a coat of primer while the POR 15 is still tacky. As you said, it is to slick and the adhesive won't bite into it. I have installed a dozen vinyl tops this way and have had no issues.
John, the top and headliner are both off and the game plan is exactly as you described; the metal has some rust but no through-and-through holes, so once the grinding is done and any filler applied and sanded, the next step will be POR15 and then primer.
