The vent definitely needs to remain open at all
times. Why the shutoff valve is there: If your boat ever did any serious cruising offshore where weather conditions can sometimes keep decks awash for days, a shutoff valve on vent lines prevents the tank from taking on water through the vents, especially if the tank is forward in the boat and the vent line runs forward. Most blue water owners have the ability to flush directly overboard at seas, so there's no risk of pressurizing the tank while the vent is closed--as long as they remember to open it again when they get into coastal waters again. Shutoffs aren't standard, so yours has to be owner installed by a PO. Unusual to find one on an aft tank, though...ever rarer to find one on a boat on inland waters...unless the location of the vent thru-hull coupled with the amount the boat heels puts the thru-hull underwater for long periods. However, that can usually be solved with a clamshell over it. I doubt if you're gonna spend much time in conditions that would require closing the vent...and besides, since you're on the Great Lakes (ok, not quite ON 'em, but between two of 'em), any previous option your boat may have had to flush overboard has either been removed or is illegal if it hasn't benn. So I'd remove the valve if I were you...and eliminate the risk of it becing closed by someone who doesn't know any better. 'Cuz flushing into a tank with a blocked vent will pressurize the tank, resulting in anything from a geyser when you open the deck fitting to an eruption in the toilet, or even a burst tank...and trying to pump out against a blocked vent is impossible--the pumpout will just pull a vacuum...a particularly strong pumpout can implode a tank.
