If you're using the new tank from Razor, it's not air tight and does not need a vented cap. (I just happened to call and ask him this very question moments ago during my install)
Anyhow, there is allowable air movement between the Low Alcohol sensor and the tank and the lid is not 100% airtight as well.
As far as the mini overflow goes (if you purchased one), there is a small hole punched in the tank just above the cap to allow the tank to vent. It works fine because I've had mine in for a few days and there has been no problem with liquid movement in/out of the mini overflow tank.
Hope this helps.