is it a vacuum leak or a vapor leak, i had a car fail because the vapor return line had cracks and was allowing vapors to leak out. so it failed the pressure test. Do you smell gas when its hot outside, and the car has been sitting. As a do it yourself person, I would inspect the ends of all the vacuum and vapor lines for cracking, and broken plastic pieces. If you find a cracked or expanded one, snip a little off the end, and put it back on with a clamp.
Another question is who told you it was a vacuum leak, and who specifically told you it was at the gas tank. That would seem like an unusual combination of things to be told. Did it pass the rest of the smog test, and exactly what was the failure. Does your engine light come on, and if so take it in and have your codes read. most auto part stores will do it for free.