Saw the post. My concern would be water intrusion, not just from condensation, but also leaving it open in the rain, or tied down in the dinghy and a heavy rain rain filling the dinghy. I didn't remember to open the tank, I probably wouldn't remember to close the vent.
Then you should keep the new tank. With the old style tanks you also have to remember to open/close the vent
I have the same exact Yamaha 15 hp 2 stroke and my bottom cylinder was not firing. It was due to a blocked lower cylinder exhaust nipple/hose and the cylinder ran great after I disconnected the hose and blew carb cleaner and air through it both ways.