Just looked at my Vado SL and saw the thru holes. You are correct screws and bolts would do the trick, best option would be nylon screws.Actually, when I lost the plug, I tried finding a threaded screw to plug the hole. That's when I realized it was not threaded but a through-hole. So, yes, I guess I could come up with screws and bolts that go through the chain stay.
My heel occasionally hits the plug and slowly pulls it out. By the way, here's what the rubber plugs look like:
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