For a V92 polyester thread and this application, then I would prefer a thicker needle that a size 18/110 like size 19, 20 or 21. In my Singer 237 manual is stated needles up to size 21, and I have used size 22 with no problem. How thick are the five layers of vinyl you intent to sew? You may encounter problems if the thickness of firm fabric increases 5 mm, because of the feed timing, and you cannot adjust that.
My best guess would be, that you need to have a stronger beehive spring in the tensioner. You can easily switch the old back. If you get problems with damaged thread, then you most likely need to modify the shuttle hook, so you get a a smooth thread U-turn near the hook. This will only make the maschine better for thinner threads too.