Easiest way would be to go to your gas tank, locate the vent pipe, loosen it from the tank and blow thru it. If you can blow air thru it that means it's not clogged, and it's ok. If it's blocked that means you should either clear the blockage or replace it. Then check the stub coming out from the tank that the vent hose attaches to. Make sure that's not blocked also.
There's nothing mysterious about a vent pipe. As the name implies, it's just a pipe or hose that allows vapors to vent in and out of your gas tank. If it's blocked then air will not be able to enter the tank and replace the gas being drawn out into your engine, so in that respect it prevents a vaccum from forming in your tank. Additionally, gas fumes will not be able to vent out of your tank on a hot day when the gas in your tank expands due to the heat, so it prevents your tank from becoming pressurized and possibly bursting.
Simply stated, it's a device that allows the inside pressure in your tank to constantly equalize with the outside atmospheric pressure.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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