There should be an inline "rollover" valve to keep the tank from draining if the bike falls over. on CA emissions bikes, the vent hose terminates in a canister about the size of a Fosters tin attached where some think a centerstand should bolt on. On 'rest of the world' bikes, it dangles down well behind the engine.
The hose should be routed around the front of the rubber tank holder to keep it from getting pinched when the tank is installed.
By the way, a major overflow while filling the tank can cause enough gas to run down the hose to close the rollover valve. The cure is to empty out the hose and give the valve a good shake or maybe blow some compressed air up from the bottom to open it.
