Canvas will breath. Vinyl will hold moisture. Look at the condensation under a plastic tarp.
That's what I figured. Once we had a barbecue grill that came with a vinyl cover, and someone told us that it will rust out faster if you keep that vinyl cover on it, due to holding the moisture in.
The problem we have had with canvas boat covers is that in our yard the covers get very stained with bird droppings and tree droppings, and mold; it is impossible to clean them, so the covers literally started looking like [censored]. We decided it is a lot easier to clean that stuff off the boat than off the canvas cover. It's probably not a problem out in an open area in the direct sun.
