A valve in the vent line that goes to a vent by the FW fill seems unusual.
Many larger TT's have vents that are simply an inverted U that come out of the bottom of the TT, rather than going back up to the FW fill connection. These units may have a valve (or the owner installs a valve) to prevent siphoning of water out of the tank while you're driving down the road. (See vent in attached pic, although it doesn't show a valve). Since the vent normally doesn't really come out of the top of the tank, but somewhere on the side, when you make a turn, the water sloshes into the vent. If it gets to the top of the U, it creates a siphon and will drain the tank down until the level is below the level of the vent connection. Some people report they've lost half a tank of water due to this. Putting a valve in the vent line prevents this siphoning effect.
In your case, since the vent goes up to the FW fill, and therefore shouldn't have an inverted U, it doesn't make sense to have one. I'd leave it open.
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