Any way you can post a picture of that?
Is the valve connected to the tank 3/4 of the way up, or its connected to the bottom of the tank but plumbed so that it sits 3/4 of the way up?
Hopefully another VCross owner can help.
It seems to me like there should only be four connections to the tank. The water outlet which should connect at the bottom and go to the pump.
The vent, which should connect at the top of the tank (with never a valve).
The fill, which will usually be a bigger hose if its gravity feed. Could be top or bottom.
The drain, which might just be a tee off the water outlet to the pump or could be its own "bottom" connection. A valve with nothing on the other side like you describe sure sounds like a drain, but it would have to be connected to the tank at the bottom.
If you can see the vent and know its not that valve, then I'd say you can close it.