The vent is the tank connection that says System Overflow in the diagram.
The original tank may say "outlet".
The instruction says connect the 20 inch small diameter hose to the outlet/overflow.
The other end of the hose simply hangs down with no connection (open to atmosphere).

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."