Never use an electric powered vacuum to suck gas/fumes. The possibility of gas fumes having an explosive meeting with the sparks inside the electric motor is just too risky.

Another option is to connect a length of clear plastic tubing to the end of the fuel line. Loosen the gas cap. First BLOW on the clear tube and listen for bubbling in the gas tank. 4 gallons should be enough to submerge the pickup. If you, or your assistant, don’t hear bubbling in the tank then you likely have an issue (leak) with the pickup tube.

If you DO hear bubbling then carefully start sucking on the clear tube. Make sure the end of the tube that you are sucking on is well elevated above the rest of the fuel system. You should be able to suck fuel part way up into the clear tube and stop before it gets in your mouth.