If we are talking about a flintlock with its vent going directly into the side of the barrel in front of the breech plug face, I see nothing wrong with plugging the vent hole while loading.
If we are talking about a flintlock with one of the typical "patent breech" or more correctly chambered breech plug, I totally disagree with plugging the vent hole while loading.
These chambered breech guns have a very small flame channel that runs from the vent location thru the breech plug to connect the chamber.
This connecting flame channel hole is typically about 1/8 inch in diameter.
On these guns, if the vent hole is left open, ramming the projectile down the bore will blow some of the powder charge thru the small connecting hole. This results in some fresh gunpowder being right at the vent. Perfect for a flintlock that requires fresh powder at the vent in order for it to fire.
Plugging the vent hole on a chambered breech style gun while ramming the projectile will result in no appreciable blast of air to move the fresh powder thru the flame channel to the vent hole. With no fresh powder at the vent, flintlocks have a very difficult time firing.