I have been following the various dialog on vent positions on Masterbuilt 30" and 40". I recently got my Masterbuilt 40" NEW GENERATION (part # 20070512)smoker and on page 3 of the manual it does talk about the vent on the side of the unit. On the OLD GENERATION version that vent is on the top. I don't have that unit or it's manual so I can't say what the manual says.
My first attempt smoking chicken thighs and legs, leaving the vent closed, for me, creates too much moisture and the chicken seemed rubber. It was over smoked , just way too smokey. The second try I left the vent open and what a difference. Mildly smokey, and very tender meat. Ditto for my St. Louis ribs, they came out great. I guess the best way for each individual is try it both ways and you will then know which works best for you.
Great Job---Didn't take you long to learn that closing your top vent will keep the smoke in the smoker until it gets stagnant. You gotta keep it flowing.