Vapor-Liquid Separator

No. The line which goes up front, and originally connected to the carbon can system up there IS THE VENT and MUST BE open to the atmosphere

...........at least if I understand what you are trying to do. If the tank has a dedicated vent line you need to use that for a VENT and it must go up high to prevent siphoning. This is why the 69/ earlier system with the vent "teed" into the filler tube worked so well. It was almost impossible to create a siphon situation with that.

Only situation I can imagine is that EG you filled it VERY full with cool gas, then parked it with the filler on the downhill side of a very steep hill.

Even then all they would do is siphon down enough to create a space in the filler neck and then "break" the siphon.