Given that otl Columbus was running short on supplies before reaching the Caribbean, if he HADNT resupplied at the Canaries, his crew might well have mutinied and returned home without ever finding the New World, no?
I think he'd been given a deadline, "Find land in x number of days or we turn the boat around, and don't try and stop us...or else!" and land turned up just before the deadline, without the canaries it's unlikely supplies would have stretched that far. Also as someone said if the canaries are in the hands of Portugal they might have decided to make life difficult for their rivals.
At this point its still another couple of years to the first Portuguese voyage to India (in '94), but with the rounding of the cape it would have been seen as only a matter of time. If Columbus turned out to be right then that potential trade route gets shot to hell, since if you can just sail across the ocean who needs such a round about route. Maybe make life a little inconvenient for the travelers? After all accidents (or the "discovery" of contraband, maybe) do happen...
As to Vinland no one really knows for certain. Greenland was of course well known (though by this point no one had heard from the colony in decades) but until the remains were found in the sixties Vinland was always treated as a myth. There may have been vague rumors about a bit of land to the West, but we just don't know if Columbus knew about it when he sailed.
About the Grand Banks, I'd say its possible fishermen made landfall on Newfoundland (if not the mainland) but as some have mentioned the Grand Banks are a fisherman's gold mine, and mentioning "We found land here," leads to;
"Oh how did you end up finding that?"
"Er, well, we were just..." and that lovely monopoly on the fishing goes out the window. It might be that Cabot's trip in 1497 was the "Official" opening up of something that had been known about for a while, once people started asking questions about all those long fishing trips...