Maybe I'm misunderstanding here... but if by "the vent" you mean the direct ducting from the bedroom AC into the room... I've always left mine open. Never heard of such nonsense as leaving it closed. That's like someone telling you to run your furnace but leave the vents closed in your house during winter.
My normal daytime routine during the summer is to set the main AC (ZONE 1) thermostat to 70, and set the bedroom AC (ZONE 2) to 68. I keep both bathroom doors closed, and leave the bedroom door open.
With the bathroom doors closed, the cool air from the bedroom augments the AC in the main room... and the bath stays cool enough with just the single ducted vent from the Zone 1 AC open, especially when I place my cardboard cover on the ceiling 'under' the skylight. (That skylight, while nice in the winter, radiates a LOT of heat in summer)