A one way vent (out) could cause tank collapse and starvation of the fuel pump. A one way vent (in) would prevent the tank from venting excess pressure. The cap should keep the tank near atmospheric pressure yet resist fuel spillage and evaporation.

My guess is the wrong part was in the box. That could have been a disaster.

Venting the vapor line would allow the tank to breathe, but would also put gasoline vapor under your hood in near proximity to your exhaust manifolds and battery. Probably not a good idea...